Forming solder balls on substrates

ABSTRACT

A mask (stencil) having cells (openings) is disposed on a surface of a heater stage, and is then filled (printed) with solder paste. Then a substrate is assembled to the opposite side of the mask. Then the solder paste is reflowed. This may be done partially inverted. Then the mask is separated from the substrate, either before or after cooling. Solder balls are thus formed on the substrate, which may be a semiconductor wafer. A biased chuck urges the substrate into intimate contact with the mask. A method for printing the mask with solder paste is described. Methods of forming high aspect ratio solder bumps (including balls and reflowable interconnect structures) are described.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of U.S. Ser. No. 10/643,766 filed Aug. 18, 2003(issuing as U.S. Pat. No. 7,007,833).

U.S. Ser. No. 10/643,766 is a continuation-in-part of:

-   -   Ser. No. 09/962,007 filed Sep. 24, 2001 (U.S. Pat. No.        6,609,652, Aug. 26, 2003) which is a continuation-in-part of:    -   U.S. Ser. No. 09/273,517 filed Mar. 22, 1999 (U.S. Pat. No.        6,293,456, Sep. 25, 2001), which is a continuation-in-part of        each of:    -   U.S. Ser. No. 08/863,800 filed 27 May 1997 (U.S. Pat. No.        5,988,487, Nov. 23, 1999);        -   U.S. Ser. No. 60/079,006 filed 23 Mar. 1998;        -   U.S. Ser. No. 60/079,221 filed 24 Mar. 1998; and        -   U.S. Ser. No. 60/092,055 filed 8 Jul. 1998,            all of which are incorporated in their entirety by reference            herein.

U.S. Ser. No. 10/643,766 is also a continuation-in-part of:

-   -   U.S. Ser. No. 10/630,310 filed Jul. 30, 2003 now abandoned as a        continuation-in-part of the aforementioned Ser. No. 09/962,007        filed Sep. 24, 2001 (U.S. Pat. No. 6,609,652, Aug. 26, 2003).

TECHNICAL FIELD OF THE INVENTION

The invention relates to methods of forming solder balls on substrateswhich are electronic components such as semiconductor devices(integrated circuit chips) and interconnection substrates, and toapparatuses for forming the solder balls on the electronic components.

BACKGROUND OF THE INVENTION

In recent years, flip-chip bonding techniques have increasingly beenused to connect (bond) integrated circuit (IC) chips to interconnectionsubstrates and to package substrates. In flip-chip bonding an IC chipcomponent to an interconnection component such as ceramicinterconnection substrate, a plurality (e.g., an array) of solder balls(also called “solder bumps”) is formed on a face of a component,typically the IC chip component, and the bumped component is broughtinto a face-to-face relationship with the other component. The twocomponents are then heated (such as in a furnace) to reflow (heat, thenallow to cool) the solder bumps, thereby making electrical connectionsbetween respective terminals of the two components.

A need for ever finer pitch arrays of solder balls has accompanied anincrease in the circuit density of IC chips and multi-chip modules. Forexample, an IC chip to be flip-chip connected to an interconnectionsubstrate may require an array of 4 mil (100 μm) diameter solder ballsdisposed at an 8 mil (200 μm) pitch.

Definitions

As used herein, the term “solder ball” refers to a substantiallyspherical or hemispherical mass (bump) of solder (e.g., lead-tin solder)resident on an substrate (e.g., electronic component), suitable forbeing re-flowed to join the electronic component to another electroniccomponent. A “large solder ball” is a solder ball having a diameter ofgreater than 20 mils (>0.020 inches). A “small solder ball” is a solderball having a diameter of up to 20 mils (<=0.20 inches).

The following units of length and their equivalents are used herein:

1 mil=0.001 inches

1 micron (μm)=0.000001 meters

25.4 μm=1 mil

1 millimeter (mm)=0.001 meters

As used herein, the term “pitch” refers to a distance between centers ofadjacent solder balls on pads of a substrate. “Coarse pitch” refers to apitch which is at least 50 mils, and connotes a “low density” of solderballs. “Fine pitch” refers to a pitch which is up to 20 mils, andconnotes a “high density” of solder balls.

For example, a typical “BGA” substrate has 30 mil diameter solder ballsdisposed at a 50 mil (coarse) pitch. A typical “μBGA” (microBGA)substrate has 15-20 mil diameter solder balls disposed at a 30 mil(“medium”) pitch. A typical “flip chip” substrate has 4-5 mil diametersolder balls disposed at an 8-100 mil pitch.

As used herein, the term “electronic component” includes any circuitizedsubstrate, typically having “pads”, including but not limited tointegrated circuit (IC) chips (including prior to or after singulationfrom a semiconductor wafer), printed circuit boards, polyimideinterconnection elements, ceramic substrates, and the like.

As used herein, a “substrate” is an electronic component having anominally flat surface upon which it is desirable to form solder ballsto effect electrical connections to another electronic component. “Wafersubstrates” are substrates (or electronic components) which aresemiconductor (crystalline, typically silicon) wafers. Any substratewhich is not a wafer substrate is an “other substrate”. Ball grid array(BGA) substrates are other substrates.

As used herein, the terms “substrate bumping” and “ball bumping” referto a process for forming solder balls on substrates. As used herein,“bumping machines” comprise equipment adapted to perform substratebumping.

Ball Bumping Techniques

A number of techniques are known for ball bumping electronic components,some of which are not well suited to fine pitch ball bumping.

In an evaporation technique, solder is evaporated through a metal maskin an evacuated chamber. This requires a high investment in capitalequipment and has high cost associated with cleaning the processingequipment and with replacing the metal mask on a frequent basis. Thermalmismatch between the evaporation mask and the substrate being ballbumped tends to limit the usefulness of the technique to moderatedensities and moderate solder bump sizes.

Electroplating techniques have been used to achieve higher densities andsmaller bump sizes. In this technique, the substrate surface is coveredwith an electroplating seed layer, then masked with photoresist which ispatterned and developed to form an electroplating mold over eachsubstrate pad. The seed layer is then electroplated, filling the molds,and the photoresist and vestigial seed layer are thereafter stripped(etched away), leaving behind the plated bumps. This technique is timeconsuming, requires high capital expenditure, and involves hazardouschemicals.

In the stenciling technique, a stencil having apertures therein isplaced over the substrate with the apertures overlying correspondingpads of the substrate. As the stencil is held in place, an amount ofsolder paste is dispensed onto the stencil, and a screening blade(sometimes called a “doctor blade”) is moved across the stencil surfacein a manner to force solder paste into the stencil apertures. Thestencil is then removed, which leaves behind bodies of solder paste onthe pads, and the bodies are thereafter reflowed to form solder bumps onthe substrate. This technique is relatively inexpensive, and comprisesonly a few quick steps, but is generally not well suited to small bumpsizes and high bump densities.

Conventional solder paste typically contains tiny particles of soldermaterial (lead/tin), in a matrix of flux, and comprises about 30% (byvolume) solid material.

U.S. Pat. No. 5,539,153 (“Hewlett Packard”), incorporated in itsentirety by reference herein, discloses a method of bumping substratesby contained paste deposition. A non-wettable metal mask (stencil) isdisposed on a substrate such that a plurality of apertures in the maskalign with a plurality of pads on the substrate. The apertures arefilled with solder paste in a manner comparable to that which wasdescribed hereinabove with respect to the stenciling technique. Thesolder paste is then reflowed with the mask in place. After reflow, themask is removed.

U.S. Pat. No. 5,492,266 (“IBM-1”), incorporated in its entirety byreference herein, discloses a process for forming solder on selectcontacts of a printed circuit board (PCB), and is generally similar tothe aforementioned Hewlett Packard Patent. A non-wettable stencil havingopenings is positioned on the board, the openings are filled with solderpaste and, with the stencil fixedly positioned on the board, the solderpaste retained by the stencil pattern is reflowed to selectively form onthe underlying contacts of the printed circuit board.

U.S. Pat. No. 5,658,827 (“IBM-2”), incorporated in its entirety byreference herein, discloses a method for forming solder balls on asubstrate. The solder balls are formed by squeegeeing solder pastethrough apertures in a fixture into contact with pads on a substrate,and heating the fixture, paste and substrate to reflow the solder pasteinto solder balls that attach to the pads and are detached from thefixture. After cooling, the fixture is separated from the substrate. Inan embodiment of the method, the fixture and substrate are inverted, andanother surface mount electrical component is placed on the oppositesurface of the substrate prior to heating the substrate.

The aforementioned Hewlett Packard, IBM-1 and IBM-2 patents all describeprinting solder paste through a mask or stencil onto a substrate, andreflowing the solder paste with the stencil in place on the substrate.In each case, the cells formed by the stencil apertures/openings areopen on one side (the side of the stencil opposite the side in contactwith the substrate). No admission is made herein that the invertedtechnique described in the IBM-2 patent would actually work asdescribed.

The aforementioned “parent” U.S. patent application Ser. No. 08/863,800(U.S. Pat. No. 5,988,487), discloses CAPTURED-CELL SOLDER PRINTING ANDREFLOW METHODS AND APPARATUSES. Generally, a screening stencil is laidover the surface of the substrate and solder paste material is depositedinto the stencil's apertures with a screening blade. The stencil isplaced in such a manner that each of its apertures is positioned over asubstrate pad upon which a solder bump is to be formed. Next, a flatpressure plate is laid over the exposed top surface of the stencil,which creates a fully enclosed (or “captured”) cell of solder materialwithin each stencil aperture. Then, with the stencil and plate remainingin place on top of the substrate, the substrate is heated to atemperature sufficient to reflow the solder material. After reflow, thesubstrate is cooled, and the pressure plate and stencil are thereafterremoved, leaving solder bumps on the substrate pads. The use of thepressure plate ensures proper formation of the solder bumps at highdensities of solder bumps (i.e., high densities corresponding to smallsolder bump sizes and small pitch distances between solder bumps).

An example of a substrate having solder balls on a surface thereof isthe Ball Grid Array (BGA) package. The advent and popularity of the BGApackage has brought with it several new package manufacturing andassembly problems. One of the more significant problems is finding anefficient, cost-effective technique for applying the solder balls to thepackage surface. The package surface is usually formed from anelectrically insulating material (e.g., printed circuit board material)with a pattern of metallized pads disposed thereupon within the package.Several methods are currently used to form solder balls on these packagepads.

One method of forming solder balls on package pads involves theapplication of solder flux to the package pads, then placing preformedsolder balls onto the package pads, either individually or en masse,with the aid of a fixture or a “pick-and-place” apparatus similar tothose used for circuit board assembly. The package is then heated to themelting point of the solder ball alloy which will then wet the metallicsurface of the pads and join thereto. This pick-and-place methodrequired the precision handling of massive qualities of solder balls. Asthe connection counts of package increase, hundreds or even thousands ofballs must be manipulated in this fashion for each package.

An alternative method of disposing solder balls on package pads involvesusing a printing or dispensing fixture to apply measured quantities ofsolder paste (a mixture of fine solder particles in a flux-containingmedium) to the package contact pads. Upon exposure to heat, the soldermelts and surface tension causes the solder to assume a generallyspherical shape. Once cooled, the spherical shapes form ball bumps(solder balls) on the package. Evidently, solder ball contacts formed inthis manner, being generally spherical, will exhibit a 1:1 aspect ratioof height-to-width. Even if hemispherical, the solder ball contacts willhave a height:width ratio on the order of 0.5:1. In certainapplications, it would be desirable that the external package contactshave a height:width ratio in excess of 1:1 (e.g., 2:1).

Another technique for disposing solder balls on package pads involvesusing printed solder paste, then placing a preformed ball, which isessentially a combination of the two techniques described hereinabove.In this technique, solder is printed onto the contact pads to form an“adhesive” on the contact pad, then a pre-formed solder ball is placedonto the contact pad and the package is heated to reflow the solderpaste, thereby joining the pre-formed solder balls to the pads.

Difficulties with any technique involving measuring or dispensingprecise quantities of solder paste on pads to form ball bumps includedealing with the rheological characteristics (elasticity, viscosity,plasticity) of the solder paste, accurately controlling the volume ofsolder paste after dispensing and reflow, and the shape of the finalball bump. The shape of the ball bump can be affected by such factors assurface tension of the molten solder and the amount of wettable exposemetal area of the contact pad.

The generally spherical shape assumed by solder balls formed asdescribed hereinabove inherently prevents the formation of “tall” (highaspect ratio) ball bumps by ordinary means. This is a limitingcharacteristic because, in certain applications, tall solder bumps canbe used to great advantage in reflow assembly (e.g., of a packagedsemiconductor device to a printed circuit board). As mentioned above, ingeneral it is difficult to form contacts with height-to-width ratios(aspect ratios) of greater than 1:1. Some techniques involving “buildingup” of solder contact height in a series of process steps have managedto produce tall (high aspect ratio) contacts, but such techniques aretypically expensive and cumbersome in high-volume production.

Consistency in the height of solder ball contacts is another criticalfactor for successful assembly of BGA type packages to circuit boards.If one or more of the solder balls are significantly shorter than others(usually due to an insufficient amount of solder paste deposited on oneor more conductive pads prior to contact formation) it becomes highlylikely that these smaller (shorter) contacts will completely miss theirmating contact pads (on the circuit board) and will fail to form anelectrical connection between the packaged semiconductor device and theunderlying substrate (e.g., printed circuit board). Hence, qualitycontrol for BGA packages is critical, since proper electricalconnections between the BGA package and the substrate to which it isassembled are formed only if each and every one of the solder ballcontacts reflows correctly and wets its associated conductive pad on thesubstrate. Defective assemblies of packages to interconnectionsubstrates can be difficult or impossible to repair after assembly ifconnections are not properly formed. Even prior to assembly, thecorrection of improperly formed solder balls on the exterior of apackage can be very difficult and involves, initially, careful qualitycontrol inspection of the ball bumps prior to assembly of the packageddevice to a substrate.

As the volume of packages produced by the aforementioned methodsincreases, the complexity of the manufacturing processes becomes anobstacle to high manufacturing rates. In order to avoid high scraprates, high machine accuracy must be maintained, raw material properties(e.g., solder paste and pad metal) must be carefully controlled, andnumerous process parameters (e.g., amount of solder paste dispensed,size of conductive pads, temperature, shape and size of ball contact)must be monitored.

Further complicating matters, in order to accommodate different packageconfigurations (e.g., different size packages, different array spacingof the ball bump contacts, etc.), it may be necessary to change numerousparts of the manufacturing equipment (tooling). Generally speaking,complicated setup and tooling changes tend to increase downtime, therebyincreasing production cost.

Information Disclosure

The following U.S. patents are cited as being of particular interest,and are incorporated in their entirety by reference herein.

DOC. NO. DATE NAME CLASS SUBCL 6,153,505 November 2000 Bolde, et al. 438613 6,126,059 October 2000 Mackay, et al. 228 9 (div of ′487) 6,109,175August 2000 Kinoshita 101 170 6,051,273 April 2000 Dalal, et al. 427 1246,008,071 December 1999 Karasawa, et al. 438 115 5,988,487 November 1999Mackay, et al. 228 254 (parent case) 5,950,908 September 1999 Fujino, etal. 228 248.1 5,934,545 August 1999 Gordon 228 191 5,877,079 March 1999Karasawa, et al. 438 613 5,842,626 December 1998 Bhansali, et al. 228180.22 5,829,668 November 1998 George, et al. 228 254 5,806,753September 1998 Bielick, et al. 228 248.1 5,782,399 July 1998 Lapastora228 41 5,773,897 June 1998 Wen, et al. 257 778 5,759,269 June 1998Cutting et al. 118 213 5,667,128 September 1997 Rohde, et al. 228 49.55,658,827 August 1997 Aulicino, et al. 228 180.22 (“IBM-2”) 5,632,434May 27, 1997 Evans, et al. 229 44.7 5,539,153 July 1996 Schwiebert, etal. 174 260 (“HP”) 5,492,266 February 1996 Hoebner, et al. 228 248.1(“IBM-1”) 5,439,164 August 1995 Hasegawa, et al. 228 194 5,366,760November 1994 Fujii, et al. 427 96 5,310,574 May 1994 Holtmann 427 585,197,655 March 1993 Leerssen, et al. 228 254 5,172,469 December 1992Onda, et al. 29 762 5,079,835 January 1992 Lam 29 840 5,014,162 January1991 Clark, et al. 361 412

BRIEF DISCLOSURE (SUMMARY) OF THE INVENTION

It is an object of the invention to provide an improved process forforming solder balls on electronic components.

Generally, according to the invention, an electronic component substrateis processed (“ball bumped”) to form a plurality of solder balls on acorresponding plurality of pads on the substrate. A mask (stencil)having a plurality of openings (cells) is disposed on the surface of aheater stage and is printed (filled with solder paste). Then, theassembly of mask and heater stage is shuttled over to a substrate havingpads (e.g., a wafer) which is in a chuck. The filled openings of themask are aligned over the corresponding plurality of pads on thesubstrate.

The mask is held in intimate contact with the heater stage and with thewafer. The cells are therefore “closed” or captured. Then the heaterstage is heated to reflow the solder paste and form solder balls. Reflowmay also be performed in an inverted or in a partially-invertedorientation. The mask may be removed from the wafer (or vice versa)while still molten.

More specifically, according to the invention claimed herein, method andapparatus are provided for forming solder bumps on a substrate having aplurality of pads on a surface thereof, comprising a biased chuckassembly which urges the substrate into positive contact with the maskso as to maintain substantially intimate contact between a surface ofthe mask and the surface of the substrate.

The process of the present invention is capable of achieving highdensities of small solder balls, and is readily scaleable to lowerdensities of large solder balls. The process proceeds relativelyquickly, with low capital expenditure equipment, and without hazardouschemicals.

The present invention provides a fast, low-cost, robust,non-capital-intensive method and apparatus for forming arrays of solderbumps at moderate to high densities on electronic components, including150 μm area arrays, 200 μm area arrays, and 250 μm area arrays, formingsolder balls at 0.5 mm pitch and at 0.8 mm pitch.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made in detail to preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. The drawings are intended to be illustrative, not limiting.Although the invention will be described in the context of thesepreferred embodiments, it should be understood that it is not intendedto limit the spirit and scope of the invention to these particularembodiments.

Certain elements in selected ones of the drawings may be illustratednot-to-scale, for illustrative clarity.

Often, similar elements throughout the drawings may be referred to bysimilar references numerals. For example, the element 199 in a figure(or embodiment) may be similar in many respects to the element 299 in another figure (or embodiment). Such a relationship, if any, betweensimilar elements in different figures or embodiments will becomeapparent throughout the specification, including, if applicable, in theclaims and abstract.

In some cases, similar elements may be referred to with similar numbersin a single drawing. For example, a plurality of elements 199 may bereferred to as 199 a, 199 b, 199 c, etc.

The cross-sectional views, if any, presented herein may be in the formof “slices”, or “near-sighted” cross-sectional views, omitting certainbackground lines which would otherwise be visible in a truecross-sectional view, for illustrative clarity.

The structure, operation, and advantages of the present preferredembodiment of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings.

FIG. 1 is an exploded cross-sectional view of a method and apparatus forforming solder balls on substrates, according to patent application Ser.No. 08/863,800 (U.S. Pat. No. 5,988,487).

FIG. 1A is an enlarged (magnified) view of the substrate (102) shown inFIG. 1, after completion of ball bumping.

FIG. 1B is an exploded cross-sectional view of an alternate embodimentof a method and apparatus for forming solder balls on substrates.

FIG. 1C is a top plan view of a substrate with bond pads.

FIG. 1D is a side cross-sectional view of the substrate of FIG. 1C, withsolder balls on the bond pads.

FIG. 1E is a side cross-sectional view of the substrate of FIG. 1C,orthogonal to the view of FIG. 1D, showing the solder balls on the bondpads.

FIG. 2A is a side cross-sectional view of another technique for formingsolder balls on a surface of a substrate.

FIG. 2B is a side cross-sectional view of another technique for formingsolder balls on a surface of a substrate.

FIG. 3A is a side cross-sectional view of an alternate embodiment of atechnique for ball-bumping a substrate, according to the invention.

FIG. 3B is a top plan view of a mask (stencil) used in the technique ofFIG. 3A, according to the invention.

FIG. 3C is a top plan view of an alternate embodiment of a mask(stencil) used in the technique of FIG. 3A, according to the invention.

FIG. 3D is a top plan view of another alternate embodiment of a mask(stencil) used in the technique of FIG. 3A, according to the invention.

FIG. 3E is a side cross-sectional view of a further step in thetechnique for ball-bumping a substrate, according to the invention.

FIG. 3F is a side cross-sectional view of a further step in thetechnique for ball-bumping a substrate, according to the invention.

FIG. 3G is a side cross-sectional view of a further step in thetechnique for ball-bumping a substrate, according to the invention.

FIG. 3H is a side cross-sectional view of a ball-bumped substrate whichhas been formed according to the invention.

FIG. 3I is a schematic illustration of a top plan view of a ball in acell of a mask, such as the mask of FIG. 3B.

FIG. 3J is a schematic illustration of a top plan view of a ball in acell of a mask, such as the mask of FIG. 3C.

FIG. 4 is a schematic diagram of a machine for ball bumping substrates,according to the invention.

FIGS. 4A-4B are schematic diagrams of a process flow for ball bumpingsubstrates, using the machine of FIG. 4, according to the invention.

FIG. 4C is a schematic diagram of an alternate embodiment of a processflow for ball bumping substrates, using the machine of FIG. 4, accordingto the invention.

FIG. 4D is a schematic diagram of an alternate embodiment of a processflow for ball bumping substrates, using the machine of FIG. 4, accordingto the invention.

FIG. 4E is a partial cross-sectional view of a substrate being bumpedaccording to the inventive technique of FIG. 4D.

FIG. 5A is a side cross-sectional view illustrating a “composite” mask,according to the invention.

FIG. 5B is a side cross-sectional view illustrating a “bridge the gap”feature of the present invention.

FIG. 5C is an exploded side cross-sectional view illustrating a“stacking masks” feature of the present invention.

FIG. 6A is a side cross-sectional view of a first step of a process offorming tall, high aspect reflowable interconnect structures, accordingto the invention.

FIG. 6B is a side cross-sectional view of a next step of the process offorming tall, high aspect reflowable interconnect structures, accordingto the invention.

FIG. 6C is a side cross-sectional view of a next step of the process offorming tall, high aspect reflowable interconnect structures, accordingto the invention.

FIG. 6D is a side cross-sectional view of a next step of the process offorming tall, high aspect reflowable interconnect structures, accordingto the invention.

FIG. 7A is a top plan view of a substrate with bond pads.

FIG. 7B is a side cross-sectional view of the substrate of FIG. 7A, withsolder bumps on the bond pads.

FIG. 7C is a side cross-sectional view of the substrate of FIG. 7A,orthogonal to the view of FIG. 7B, showing the solder bumps on the bondpads.

FIG. 7D is a top plan view of a substrate with bond pads.

FIG. 7E is a side cross-sectional view of the substrate of FIG. 7D, withsolder bumps on the bond pads.

FIG. 7F is a side cross-sectional view of the substrate of FIG. 7D,orthogonal to the view of FIG. 7B, showing the solder bumps on the bondpads.

FIG. 7G is a top plan view of a substrate with bond pads.

FIG. 7H is a side cross-sectional view of the substrate of FIG. 7G, withsolder balls on the bond pads.

FIG. 7I is a side cross-sectional view of the substrate of FIG. 7G,orthogonal to the view of FIG. 7B, showing the solder balls on the bondpads.

FIG. 8A is an exploded side cross-sectional view of an alternateembodiment of a mask mounting technique, according to the invention.

FIG. 8B is a partially exploded side cross-sectional view of a techniquefor capturing the cells of the mask illustrated in FIG. 8A.

FIG. 9 is an exploded side cross-sectional view of a chuck assembly forholding a substrate which is a semiconductor wafer, according to theinvention.

FIG. 9A is a magnified cross-sectional view of a component of the chuckassembly of FIG. 9.

FIG. 10 is a schematic side view of a ball bumping machine of thepresent invention.

FIG. 10A is a schematic side view of a heater stage of the presentinvention.

FIG. 10B is a schematic cross-sectional view of a chuck assembly of thepresent invention.

FIG. 10C is a schematic cross-sectional view of a wafer being ballbumped, partially inverted, according to the invention.

FIG. 10D is a plan view of a mask, and its mounting arrangement,according to the invention.

FIGS. 11A-11C are schematic diagrams of a process flow for ball bumpingsubstrates, using the machine of FIG. 4, according to the invention.

FIG. 12 is a side, cross-sectional view of a technique for applyingsolder paste to cells in a mask, according to the invention. See FIG. 2of U.S. Ser. No. 10/630,310 filed Jul. 30, 2003.

FIG. 13 is a side, cross-sectional view of a set of blades, such asthose shown in FIG. 12, according to the invention. See FIG. 3 of U.S.Ser. No. 10/630,310 filed Jul. 30, 2003.

FIG. 14 is a schematic side view of two sets of blades, such as thoseshown in FIG. 12, according to the invention. See FIG. 4 of U.S. Ser.No. 10/630,310 filed Jul. 30, 2003.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a technique 100 for forming solder balls on a surfaceof a substrate 102, such as is set forth in “parent” U.S. patentapplication Ser. No. 08/863,800 (U.S. Pat. No. 5,988,487, Nov. 23,1999), incorporated in its entirety by reference herein.

The substrate 102 has number of pads 104 on its top (as viewed) surface.The pads 104 are typically arranged in an array, having a pitch(center-to-center spacing from one another). The substrate 102 isdisposed atop a heater stage 106.

A mask (stencil) 110 is provided. The mask 110 is a thin planar sheet ofrelatively stiff material, such as molybdenum, having a plurality ofopenings (cells) 112, each corresponding to a pad 104 whereupon it isdesired to form a solder ball on the substrate 102.

The mask 110 is placed on the top (as viewed) surface of the substrate102 with the cells 112 aligned over the pads 104. The cells 112 in themask 110 are filled with solder material 114. This is done in anysuitable manner such as by smearing solder material on the top (asviewed) surface of the mask 110 and squeegeeing the solder material 114into the cells 112 of the mask 110.

A typical solder paste contains particles of lead/tin solder, in amatrix of flux, with the following proportions: 80% (by weight) solidmaterial (e.g., particles of lead/tin solder), and 20% (by weight) flux(including volatiles). In terms of relative volume percentages, the sametypical solder paste may contain approximately 55% (by volume) of solidmaterial (metal) and 45% (by volume) of flux. As discussed in greaterdetail hereinbelow, it is preferred that a “solder material” be used inlieu of regular solder paste.

It is within the scope of the invention that the cells 112 in the mask110 are filled with solder material prior to placing the mask 110 on thetop surface of the substrate, in which case the solder-material-filledcells 112 would be aligned over the pads 104.

A pressure plate 120 is disposed onto the top (as viewed) surface of themask 110. This holds the mask 110 down onto the substrate 102, and thesubstrate 102 down onto the heater stage 106. This also closes off thecells 112—hence, the terminology “captured cell”.

The heater stage 106 is heated up, typically gradually, to a temperaturesufficient to cause the solder material in the cells 112 to melt(reflow). When the solder material melts, the individual solderparticles will merge (flow) together and, due to surface tension, willtry to form (and, typically, will form) a sphere.

When the solder material re-solidifies, it assumes a general sphericalor hemispherical shape. The mask 110 is then removed from the substrate102.

FIG. 1A is an enlarged (magnified) view of the substrate 102 shown inFIG. 1, after completion of ball bumping. Herein it can be observed thatthe solder balls 130 are generally spherical, have a diameter “D” andhave a height “H”. See also FIGS. 1C, 1D, 1E, below.

The aforementioned “parent” U.S. patent application Ser. No. 08/863,800(U.S. Pat. No. 5,988,487) describes exemplary substrate heating programs(profiles, recipes) in terms of temperature as a function of time.

A drawback of the technique 100 is that no provision is made for “outgassing” of volatiles when the solder material is reflowed.

Another drawback of the technique 100 is that heat is directed throughthe substrate 102.

FIG. 1B illustrates an alternate technique 150 (compare 100) for formingsolder balls on a surface of a substrate 152 (compare 102).

The substrate 152 has number of pads 154 (compare 104) disposed on itstop (as viewed) surface. The substrate 152 is disposed atop a chuck(base) 158, rather than atop a heater stage (106).

A mask (stencil) 160 (compare 110) having cells 162 (compare 112) filledwith solder material 164 (compare 114) is disposed on the surface of thesubstrate 152 with the cells 162 aligned with the pads 154. The cells162 may be pre-filled or filled with the mask 160 atop the substrate152.

A pressure plate 170 (compare 120) is disposed onto the top (as viewed)surface of the mask 160. This holds the mask 160 down onto the substrate152, and the substrate 152 down onto the chuck base 158. This alsocloses off the tops of the cells 162.

A heater stage 156 (compare 106) is disposed onto the top (as viewed)surface of the pressure plate 170. The heater stage 106 is heated up,typically gradually, to a temperature sufficient to cause the soldermaterial in the cells 162 to reflow. When the mask 160 is removed,solder balls such as those (130) shown in FIG. 1A will be present on thepads 154.

A drawback of the technique 150 is that no provision is made for “outgassing” of volatiles when the solder material is reflowed. However, incontrast to the technique 100, the technique 150 directs heat throughthe pressure plate 170 rather than through the substrate 152.

FIG. 2A illustrates a technique 200 for forming solder balls on asurface of a substrate 202. The substrate 202 has a top surface 202 aand a bottom surface 202 b.

In this example, forming solder balls on an external surface ofsubstrate (or board) which is a BGA substrate (board) is discussed asexemplary of forming solder balls on (ball-bumping) a substrate. Itshould, however, be understood that the techniques described herein haveapplicability to ball bumping other substrates, such as semiconductorwafers.

A typical BGA substrate 202 has a plurality of contact pads 204 on itssurface, each of which measures 35 mils across. In the typical case ofround contact pads, each pad would be 35 mils in diameter. These contactpads 204 are typically spaced 50 mils (center-to-center) apart from oneanother. Often, the pad-surface 202 a of the substrate is covered bythin (e.g., 2 mil) layer of insulating material 206, such as a polymer,which has openings 208 aligned with (centered over) the pads 204. Theinsulating material 206 has a top surface 206 a.

The openings 208 in the insulating material 206 are typically somewhatsmaller in size (area) than the pads 204—for example, each openingmeasuring only 30 mils across. Evidently then, the top surface 202 a ofthe BGA substrate 202 will be quite irregular, exhibiting peaks wherethe insulating material 206 overlaps the pads 204 and valleys betweenthe pads 204.

A mask (stencil) 210 is provided. The mask 210 is a thin (e.g., 30 milsthick) planar sheet of relatively stiff material, such as molybdenum,having a plurality of openings (cells) 212, each corresponding to a pad204 whereupon it is desired to form a solder ball on the substrate 202.A typical cross-dimension for a cell 212 is 40 mils across.

In a first step of forming solder balls on (ball bumping) the substrate202, the mask 210 is placed on the top surface 202 a of the BGAsubstrate 202 with the cells 212 aligned over the pads 204, moreparticularly, over the openings 208 in the layer of insulating material206. As illustrated, due to the size (diameter) of the cells 212, andthe irregular surface 206 a of the insulating material 206, there willbe gaps 214 between the mask 210 and the insulating material 206. Atypical dimension for the gap is 1-2 mils. As will be evident, thesegaps 214 have benefits and disadvantages.

In a next step of forming solder balls on the substrate 202, the cells212 in the mask 210 are filled with solder material 220 which is shownas a number of various-size spheres. (The middle cell 212 in the figureis shown without solder material 220, for illustrative clarity.) This isdone in any suitable manner such as by smearing solder past on the topsurface 210 a of the mask 210 and squeegeeing the solder material 220into the cells 212 of the mask 210.

It is within the scope of the invention that the cells in the mask arefilled with solder material prior to placing the mask 210 on the topsurface 202 a of the BGA substrate 202 with the (filled) cells 212aligned over the pads 204.

In a next step of ball bumping the substrate 202, a heater stage(platen) 230 is disposed onto the top surface 210 a of the mask 210, andthe substrate 202, mask 210 and heater stage 230 are held together withclamps (not shown), in the orientation shown in the figure—namely, withthe heater stage 230 on top of the mask 210, and with the mask 210 ontop of the substrate 202.

It is within the scope of the invention that a pressure (contact) plate(not shown, compare 170) is disposed on the top surface 210 a of themask 210, between the heater stage 230 and the mask 210.

In a next step of forming solder balls on the substrate 202, the heaterstage 230 is heated up, typically gradually, to a temperature sufficientto cause the solder material 220 to melt within the cells 212. When thesolder material 220 melts, the individual solder particles will merge(flow) together and, due to surface tension, will try to form (and,typically, will form) a sphere.

During reflow heating, small-sized solder particles within the soldermaterial can “leak” out of the gap 214. This is not desirable. On theother hand, the gap 214 allows volatile material to “out gas”.

After reflowing the solder material 206, the heater stage 230 is eitherremoved immediately, so that the solder can cool down, or is kept inplace and allowed to cool down until the solder has re-solidified assolder balls. As described in greater detail hereinbelow, often, as thesolder material cools off, it will try to form a ball which has a largerdiameter than the cell. This results in (i) there being an interferencefit between the resulting solder ball and the sidewalls of the cell and(ii) a deformed solder ball. Regarding the latter, it is known to reflowthe resulting deformed solder balls after removing the mask in order tocause them to assume a more spherical shape.

The forming of solder balls (240) on a substrate (202) is suitablycarried out in the orientation illustrated in FIG. 2A—namely, the mask(214) is disposed on top of the substrate (202) and the heater stage(230) is disposed on top of the mask (214).

Alternate embodiments of the invention, where reflow heating is carriedout with the mask/substrate assembly inverted, or partially inverted,are described hereinbelow.

An inherent “side-effect” of the described technique 200 is that theflux material in the solder material (106) will liquefy and may run downonto the top surface 202 a of the substrate 202 or, in the case of therebeing an insulating layer 206, onto the top surface 206 a of theinsulating layer 206. In that the ball-bumped BGA substrate (orball-bumped semiconductor package assembly) may be “warehoused” formonths, prior to being mounted to an interconnection substrate, it isknown that it should be cleaned of flux (de-fluxed) soon after thesolder balls have been formed on the pads (204). Furthermore, whateverflux was present in the solder material (220) will largely have beendissipated (run-off and cleaned off) in the process the flux ran off(and cleaned) off the solder balls, resulting in that they will need tobe re-fluxed prior to assembling to the interconnection substrate.Typically, the flux component of solder material will lose its viscosityand start running at a much lower temperature than the melting point ofthe solid particulate (solder) component of the solder material.

FIG. 2B illustrates another prior art technique 250 (compare 200) forforming solder balls on a surface of a substrate 252 (compare 202)—morespecifically on contact pads 254 (compare 204) of a substrate 252. Thesubstrate 252 has a top surface 252 a (compare 202 a) and a bottomsurface 252 b (compare 202 b), contact pads 254 (compare 204) disposedon its top surface 252 a, and a thin layer of insulating material 256(compare 206) which has openings 258 (compare 208) aligned with(centered over) the pads 254. The insulating material 256 has a topsurface 256 a (compare 206 a).

A mask 260 (compare 210) has a plurality of cells 262 (compare 212). Inthis example, the cross-dimension of a cell 262 is smaller than in theprevious example (for example only 25 mils across).

Due to this smaller cross-dimension, a gap (compare 214) is not formedbetween the mask 260 and the insulating material 256, and the mask 260is essentially “sealed” to the substrate 252. This has the advantagethat small solder balls and flux material will not “leak out” (throughthe gap) onto the surface of the substrate 252 (except in the case thatthe mask is held off of the surface of the substrate by a defect or bycontamination). However, the lack of a gap also means that volatileshave no place to escape (vent, “out gas”). Thus, the rate at which thetemperature of the solder material 270 is elevated becomes critical.More particularly, if the solder material is heated too fast, thevolatiles will try to escape the cell (262) in a “violent” manner, oftentending to lift the mask 260 off of the substrate 252. This is notdesirable.

As in the previous example, in a first step of forming solder balls onthe substrate 252, the mask 260 is placed on the top surface 252 a ofthe BGA substrate 252 with the cells 262 aligned over the pads 254, moreparticularly, over the openings 258 in the layer of insulating material256.

As in the previous example, in a next step of forming solder balls onthe substrate 252, the cells 262 in the mask 260 are filled with soldermaterial 270 (compare 220) which is shown as a number of various-sizespheres. (The middle cell 262 in the figure is shown without soldermaterial 220, for illustrative clarity.)

As in the previous example, it is within the scope of the invention thatthe cells 262 in the mask 260 are filled with solder material prior toplacing the mask 260 on the top surface 252 a of the BGA substrate 252with the (filled) cells 262 aligned over the pads 254.

As in the previous example, in a next step of forming solder balls onthe substrate 252, a heater stage (platen) 280 (compare 230) is disposedonto the top surface 260 a of the mask 260, and the substrate 252, mask260 and heater stage 280 are held together with clamps (not shown), inthe orientation shown in the figure—namely, with the heater stage 280 ontop of the mask 260, and with the mask 260 on top of the substrate 252.

It is within the scope of the invention that a pressure (contact) plate(not shown, compare 170) is disposed on the top surface 260 a of themask 260, between the heater stage 280 and the mask 260.

As in the previous example, in a next step of forming solder balls onthe substrate 252, the heater stage 280 is heated up (gradually, asnoted hereinabove), to a temperature sufficient to cause the soldermaterial 270 to melt within the cells 262. When the solder material 270melts, the individual solder particles will merge (flow) together and,due to surface tension, will try to form (and, typically, will form) asphere.

As in the previous example, after reflowing the solder material 270, theheater stage 280 is either removed immediately, so that the solder cancool down, or is kept in place and allowed to cool down until the solderhas re-solidified as solder balls.

As described in greater detail hereinbelow, often, as the soldermaterial cools off, it will try to form a ball which has a largerdiameter than the cell. This results in (i) there being an interferencefit between the resulting solder ball and the side walls of the cell and(ii) a deformed solder ball. Regarding the latter, it is known to reflowthe resulting deformed solder balls after removing the mask in order tocause them to assume a more spherical shape.

As in the previous example, the forming of solder balls on a substrate(252) is typically carried out in the orientation illustrated in FIG.2B—namely, the mask (260) is disposed on top of the substrate (252) andthe heater stage (280) is disposed on top of the mask (260).

Alternate embodiments of the invention, where reflow heating is carriedout with the mask/substrate assembly inverted, or partially inverted,are described hereinbelow.

A benefit of the techniques 200 and 250 shown in FIGS. 2A and 2B is thatthe mask and the solder material contained within the cells of the maskare heated essentially directly, rather than through the substrate aswas the case with the technique 100 shown in FIG. 1. Also, as shown inFIG. 2A, a gap 214 allows for out gassing, which permits faster reflowtimes.

FIG. 3A illustrates a technique 300 (compare 100, 200, 250) for ballbumping a substrate 302 (compare 102, 202, 252)—more specifically oncontact pads 304 (compare 104, 204, 254) of a substrate 302. It iswithin the scope of this invention that the substrate 302 is anyelectronic substrate, including a semiconductor wafer or a BGA board.The substrate 302 has a top surface 302 a (compare 102 a, 202 a, 252 a)and a bottom surface 302 b (compare 102 b, 202 b, 252 b). A plurality ofcontact pads 304 (compare 104, 204, 254) are disposed on the top surface302 a of the substrate 302, and are covered by a thin layer 306 (compare206, 256) of insulating material, such as a polymer, (or, in the case ofthe substrate 302 being a semiconductor wafer, a passivation layer)which has openings 308 (compare 108, 208, 258) aligned with (centeredover) the pads 304. The insulating material 106 has a top surface 306 a(compare 106 a, 206 a, 256 a). The top surface 302 a of the substrate302 has an irregular topology, exhibiting peaks where the insulatingmaterial 306 overlaps the pads 304 and valleys between the pads 304.

A mask (stencil) 310 (compare 110, 210, 260), which is suitably a thinplanar sheet of relatively stiff material, such as molybdenum, has aplurality of cells 312 (compare 112, 162, 212, 262), each correspondingto and aligned with a pad 304 whereupon it is desired to form a solderball on the substrate 302. The cells 312 in the mask 310 may be round(circular), as illustrated by the array of cells 312 b in FIG. 3B.Preferably, however, the cells are not round (circular). For example, asillustrated by the array of cells 312 c in FIG. 3C, the cells 312 c maybe square. In this manner, for a given spacing, e.g., 10 mils betweenthe peripheries of adjacent cells 312 c (in other words the size of the“web” in the mask between adjacent cells 312 c), each individual cell312 c can have a larger area, hence a larger volume for a giventhickness mask, than a round cell (312 b). Alternatively, as illustratedby the array of cells 312 d in FIG. 3D, the cells 312 d may have atrapezoidal shape, and be arranged in alternating orientations. As inthe example of square cells (See FIG. 3C), in this manner, for a givenspacing, e.g., 10 mils between the peripheries of adjacent cells 312 d(in other words the size of the “web” in the mask between adjacent cells312 d), each individual cell 312 d can have a larger area, hence alarger volume for a given thickness mask, than a round cell (312 c). Allother things being equal, the volume of a trapezoidal cell (312 d) canbe greater than that of a square cell (312 c) which, in turn in greaterthan that of a round cell (312 a). Non-round cells (e.g., 312 c and 312d) in a mask (e.g., 310) for forming solder balls on a surface of asubstrate is considered to be within the scope of the invention. Itshould be noted that FIGS. 3B, 3C and 3D are not drawn to the same scaleas FIG. 3A.

Returning to FIG. 3A, in a first step of forming solder balls on thesubstrate 302, the mask 310 is placed on the top surface 302 a of thesubstrate 302 with the cells 312 (preferably the cells 302 c or 302 d)aligned over the pads 304. Evidently, the irregular surface 306 a of theinsulating material 306 will result in there being gaps 314 (compare114) between the mask 310 and the insulating material 306. These gaps314 can perform a beneficial purpose of allowing volatiles to vent (outgas).

The mask 310 is held in any suitable manner either in directface-to-face contact with the substrate 302, or ever so slightly spacedtherefrom.

Then, the cells 312 are filled with solder material 320. (The middlecell 312 in the figure is shown without solder material 320, forillustrative clarity.)

It is within the scope of the invention that the cells 320 of the mask310 are filled with solder material either when the mask is inface-to-face contact with the substrate 302, or “off-line” (prior tobringing the mask into face-to-face contact, or near contact, with thesubstrate.

At this point in the process, the technique of the present inventiondeviates significantly from the techniques (100, 200, 250) describedhereinabove.

FIG. 3E illustrates a next step in the process, wherein the assembly ofthe mask 310 and the substrate 302, with solder material 320 loaded intothe cells 312 of the mask 310 is inverted, so that the substrate 302 isphysically atop above the mask 310, as is illustrated in the figure. Inthis “upside-down” orientation, the solder material 320 will not fallout of the cells 312 in the mask 310, because it is “sticky”, being acombination of solid particles and relatively viscous (at roomtemperature) flux material. The solder material 320 has the generalconsistency of toothpaste. It should be noted that in this figure (FIG.3E) the middle cell 312 is shown filled with solder material 320.

Alternatively, it is within the scope of the invention that a pressure(or “contact”) plate is placed against the mask, as described withrespect to other embodiments of the invention.

As illustrated, this upside-down assembly of the mask 310 and thesubstrate 302, with solder material 320 loaded into the cells 312 of themask 310 is brought into contact with a heater stage 330 (compare 130,230) which is either brought up to or which has been pre-heated to atemperature which is greater than the melting point of the solidparticles in the solder material 320.

It is generally preferred that the solder material is gradually ratherthan abruptly reflowed. For example, by bringing its temperature up toless than its melt point to allow it to “condition” prior to causing itto reflow. It is within the scope of the invention that any suitableheat profile can be used.

For example, “63/37” lead/tin solder has a melting temperature ofapproximately 183° C. (Centigrade). In which case, the heater stage 330may be preheated to 140° C.-150° C. for conditioning the soldermaterial, then brought up to a temperature of at least 215° C.,preferably to a temperature which is 20° C.-40° C. higher than themelting temperature of the solid particles of the solder material (i.e.,the heater stage 330 is preferably heated to approximately 220° C.-225°C. for reflowing the aforementioned 63/37 solder material).

The upside-down assembly of the mask 310 and the substrate 302, withsolder material 320 loaded into the cells 312 of the mask 310 is held incontact with a heater stage 330 for a sufficient period of time “t” forthe solid particles in the solder material 320 to melt, and preferablynot much longer. Given the dynamics of the overall system, this periodof time “t” is preferably determined empirically. However, since theheater stage 330 was already preheated, and since the solder material320 and the solder mask 310 are both fairly good conductors of heat, andbased on experimental trials of the technique of the present invention,it is contemplated that, for most anticipated microelectronicapplications of the present invention, a period of time “t” of 5-20seconds will be sufficient time for the solder material 320 to liquefy.However, in the case of a board (substrate) having heat sinks, forexample a thick copper heat sink, the time “t” required to form thesolder balls on the substrate may more than 20 second, for example 30seconds.

FIG. 3F illustrates a next step of the process wherein, after the solidparticles in the solder material 320 have liquefied, the heater stage330 is removed from being in further contact with the upside-downassembly of the mask 310 and the substrate 302. This can be done eitherby lifting the upside-down assembly of the mask 310 and the substrate302, or by lowering the heater stage 330. The liquefied solder particlesof the solder material 320 will begin to cool off and coalesce into onesolid mass, typically generally in the form of a sphere.

FIGS. 3G and 3H illustrate the solder balls 340 that are formed by theprocess of the present invention described hereinabove. In FIG. 3G, themask 310 is still in place. In FIG. 3H, the mask has been removed, andthe ball bumped substrate has been re-flipped over.

While FIG. 3G illustrates an “ideal” situation where the resultingsolder balls are perfectly centered within their respective cells, thereal world tends not conform so neatly to perfection. As illustrated inthe schematic illustration of FIG. 3I, a solder ball 342 which isslightly off-center in a round cell 344 (compare 312 b) will exhibit anarcuate area of contact with the side wall of the cell. In contrastthereto, as illustrated in the schematic illustration of FIG. 3J, asolder ball 346_which is slightly off-center in a square cell 348(compare 312 c) will exhibit only minimal (e.g., point) contact with theside wall of the cell. The cumulative effect of a number of solder ballsmisaligned with the mask openings (cells) and being in contact with themask can have an adverse undesirable effect on subsequent separation ofthe mask from the substrate.

A benefit of this “inverted” embodiment of the present invention isthat, due to the influence of gravity (i.e., the earth's pull on objectstowards the center of the earth), flux material within the soldermaterial 320, which also has been liquefied, will run down the surfaceof the solid mass, rather than up to the surface of the substrate 302.This is in marked contrast to the previous examples wherein it wasobserved that the tendency was for the liquefied flux to run down ontothe substrate (102, 202, 252). This has some important beneficialresults, including:

-   -   the substrate (board) 302 does not need to be cleaned;    -   the resulting solder balls 340 are “pre-fluxed”; and    -   the resulting solder balls 340 have a clean, oxide-free surface        for better (subsequent) soldering.

Another benefit is that the resulting solder balls 340 will have aheight (diameter) which is greater than the thickness of the mask 310.Generally, large solder balls 340 having approximately a 1:1 aspectratio (height:width) are readily formed on pads of substrates using thetechnique of the present invention. As a result, the molten solder ballcan join itself to the substrate without there needing to be any directcontact between the mask and the substrate. Also, the mask can beremoved while the solder is still molten, thereby greatly facilitatingmask/substrate separation.

FIG. 4 illustrates major components of a “bumping” machine 400 for ballbumping substrates, both in the manner described hereinabove as well asusing alternate techniques. The machine 400 comprises a stable platform402.

The machine 400 comprises a chuck 404 which is disposed on the platform402, for holding a substrate 406. (The substrate 406 is not a componentof the machine 400.)

The machine 400 comprises a mask holder 408 for holding a mask (notshown), and which is mounted in an articulated manner to the platform402 so that it can be moved from a one position to another position.

The machine 400 comprises a pressure plate holder, such as a simpleframework, for holding a pressure plate 410 (compare 120), and which ispreferably mounted in an articulated manner to the platform 402 so thatit can be moved from a one position to another position. In use, it ispreferred that the pressure plate be held in intimate contact with thesurface of the mask opposite the substrate during reflow of the soldermaterial in the mask.

A heat source 412 is provided for reflowing solder material in the mask,and which is preferably mounted in an articulated manner to the platform402 so that it can be moved from a one position to another position. Theheat source 412 may be a heater stage, or may be a radiant (e.g.,infrared) heat panel, such as may be obtained from Watlow Electric Mfg.Co., St. Louis, Mo., USA.

A print station 414, which may be a flat, non-wettable surface, isoptionally provided, for off-wafer filling of the cells of the mask withsolder material, as mentioned hereinabove.

One having ordinary skill in the art to which the invention most nearlypertains will understand how to implement the machine 400, forperforming the various techniques described herein, in light of thedescriptions set forth herein.

Inverted Reflow, Inverted Cooling

FIGS. 4A-4B illustrate a technique 420 for ball bumping substrates. Inthis technique, the pressure plate is positioned above the heat source,at a location on the machine platform, as illustrated. The mask ispositioned on the substrate, which is positioned on the chuck, atanother location on the machine platform. With the mask positioned onthe substrate, the mask cells may be filled with solder material. Next,the assembly of the chuck/wafer/mask are shuttled into position, upsidedown, on the pressure plate. The heat source is turned on, and thesolder material in the mask melts. Then the heat source is shut off toallow the solder material to cool and coalesce into solder balls.Finally, the mask is separated from the substrate and the substrate isseparated from the chuck.

It should be noted that in this, as well as in certain other embodimentsdescribed herein, that heat must pass through the pressure plate to meltthe solder material within the mask. In the case of using a heat sourcewhich is an infrared-type heat source, a quartz pressure plate may beused. Otherwise, the pressure plate may be molybdenum, stainless steel,or the like.

It is within the scope of the invention that the mask cells may bepre-filled with solder material, such as by positioning the mask on aprint station surface (414, described hereinabove), or by utilizing thepressure plate as a print station (in which case, the heat source shouldnot be “on”).

It is within the scope of the invention that the heat source may have aflat surface so that it can perform the function of the pressure plate,without an additional component.

Inverted Reflow Un-Inverted Cooling

FIG. 4C illustrates a technique 440 for ball bumping substrates. Thistechnique proceeds in the manner of the technique 420 describedhereinabove, up to the point of melting the solder material with thesubstrate inverted, as illustrated in FIG. 4B. Then, rather thanallowing the solder material to cool in this orientation, the assemblyof the chuck/wafer/mask are repositioned away from the heat source sothat the wafer is “right side up” (un-inverted), and the solder materialis allowed to cool. Finally, the mask is separated from the substrateand the substrate is separated from the chuck.

It is within the scope of the invention that the heat source “follows”the assembly of chuck/wafer/mask when it is repositioned, in which caseit would be switched “off” to allow the solder material to cool.

Partially-Inverted Reflow and Cooling

As mentioned hereinabove with respect to the technique 300, an advantageof reflowing the solder material in the inverted position, as describedby the techniques 420 and 440 is that out gassing may occur in gaps(e.g., 314) between the mask and the substrate, thereby permittingrelatively rapid heating (melting) of the solder material. However, itis possible that oxides may become trapped in the interface between thesolder material and the substrate pad when reflowing in the invertedorientation.

FIGS. 4D and 4E illustrate an alternate technique 460 for ball bumpingsubstrates. In this technique, rather than inverting the substrate (from180° to 0°) to reflow the solder material, the substrate is positionedat an angle between 90° (on its side) and 0° (inverted), such as at 45°from inverted, as illustrated. (This also includes orientations for thesubstrate which are beyond inverted, such as −45°.) As illustrated, thesubstrate has been rotated 135° from being face (pads) up to beingpartially face down.

As best viewed in FIG. 4E, a mask 462 (compare 310) having openings(cells) 464 (compare 312) extending from a one surface to an oppositesurface thereof and filled with solder material 466, has its one surfacedisposed against a surface of a substrate 468 having pads 470. Apressure plate 472 is disposed in intimate contact against the oppositesurface of the mask 462. A middle one of the cells 464 is illustratedwithout solder material 466, for illustrative clarity, so that the gap474 (compare 314) can clearly be seen. The assembly of substrate 468,mask 462 and pressure plate 472 are oriented as shown, and it can beseen that the gap 474 is at the highest point of the cell. Thisfacilitates out gassing of volatiles during reflow. The chuck and theheat source are omitted from the view of FIG. 4E, for illustrativeclarity.

This technique proceeds in the manner of the techniques 420 and 440described hereinabove, up to the point of securing the solder-laden maskto the substrate and mounting the pressure plate to the assembly. Then,the assembly is positioned as shown, partially inverted, so that acorner of each cell is the highest point in the cell (see the corner atthe gap 474). Reflow is performed in this position, using the heatsource (not shown). Finally, the mask is separated from the substrateand the substrate is separated from the chuck.

It is within the scope of the invention that rather than allowing thesolder material to cool in the partially-inverted orientation, theassembly of the chuck/wafer/mask are repositioned away from the heatsource so that the wafer is “right side up” (un-inverted, 180°), and thesolder material is allowed to cool.

It is within the scope of the invention that the heat source “follows”the assembly of chuck/wafer/mask when it is repositioned, in which caseit would be switched “off” to allow the solder material to cool.

Composite Mask and Pressure Plate

The benefit of using a pressure plate to capture the solder material inthe cells of the mask has been discussed hereinabove. It is generallypreferred that the pressure plate be intimately held against the mask sothat there are no gaps for leakage, particularly when reflowing invertedor partially inverted.

According to an aspect of the invention, a composite mask performing thefunctions of a mask and a pressure (contact) plate are formed as anintegral unit, thereby assuring no leakage between the two.

FIG. 5A illustrates an embodiment of a composite mask 500, according tothe present invention. The composite mask 500 is a rigid planarstructure having two portions—a mask portion 510 comparable (e.g.) tothe mask 110 described hereinabove, and a pressure plate portion 520comparable to the pressure plate 120 described hereinabove. A pluralityof cells 512 (compare 112) extend from a one surface of the compositemask 500, through the mask portion 510, to the pressure plate portion520. These “blind hole” type openings 512 are filled with soldermaterial 514 (compare 114) in the manner described hereinabove.

The composite mask 500 is suitably formed of a sheet of metal, such asmolybdenum, which is etched to have cells 512 extending into a surfacethereof (but not all the way through the sheet). Alternatively, thecomposite mask 500 can be formed from a sheet of metal comprising thepressure plate portion 520, a surface of which is masked, patterned, andplated up to form the mask portion 510 (with cells 512).

Alternatively, a composite-type mask can be formed from a discrete maskwelded or otherwise intimately joined (including adhered) to a discretepressure plate.

Bridging a Gap

An interesting feature/capability of the present invention isillustrated in FIGS. 5A and 5B, but is not limited to the use of acomposite mask. The composite mask 500 is illustrated disposed beneath asubstrate which is in an inverted position, for example the substrate302 from FIG. 3A (see also FIG. 3E). Note that no part of the substrate302 actually is in contact with the composite mask 500—rather, thatthere is a small gap 524 between the opposing faces of the substrate andthe mask.

As best viewed in FIG. 5B, when the solder material 514 reflows andforms a ball, the ball has a diameter (height) which is greater than thethickness of the mask (in this illustrative case, greater than thethickness of the mask portion 510 of composite mask 500), so it sticksout of the mask, “bridges” the gap 524, and wets itself to the pad 304on the substrate 302. The solder ball does this while it is in a liquidstate, at which point the mask can easily be separated from thesubstrate, thereafter allowing the solder ball to cool off (solidify).

Stacked Masks

FIG. 5C illustrates a mask stack 550 comprising a first or “Liftoff”mask 552 (compare 110) having a plurality of cells 554 (compare 112) anda second or “volume control” mask 556 having a plurality of cells 558.For example, the mask 552 is 4 mil thick, and the mask 556 is 3 milthick. The cells 558 are tapered, as illustrated, to provide reducedhole volume control. The orientation of the mask stack 550 as it wouldbe employed for ball bumping a substrate is illustrated by the substrate560 having pads 562 and a pressure plate 564.

The mask stack 500 is beneficial in applications where particularly tall(high aspect ratio) solder balls (columns) are desired to be formed on asubstrate, tending to overcome inherent limitations in the aspect ratioof holes that can be formed in masks. The two (or more) masks may beremoved one at a time after solder ball formation to reduce liftoffstress.

There have thus been described, with respect to FIGS. 5A, 5B and 5C anumber of mask “variations”, including a composite mask, a mask which isspaced from the substrate being bumped, and a mask stack. Other maskvariations may occur to one having ordinary skill in the art to whichthe present invention most nearly pertains, in light of the teachingsset forth herein.

High Aspect Ratio Ball Bumps

Solder balls which are generally spherical, will, by definition, exhibitsubstantially a 1:1 aspect (height:width) ratio. If they arehemispherical, the solder balls will have an aspect ratio ofapproximately 0.5:1. The generally spherical shape assumed by solderballs formed as described hereinabove is based on the physics of surfacetension, and inherently prevents the formation of “tall” (high aspectratio) ball bumps by ordinary means. This is a limiting characteristicbecause, in certain applications, tall (high aspect ratio) solder bumpscan be used to great advantage in reflow assembly (e.g., of a packagedsemiconductor device to a printed circuit board). As mentioned above, ingeneral it is difficult to form contacts with aspect ratios of greaterthan 1:1. Some prior art techniques involving “building up” of soldercontact height in a series of process steps have managed to produce tall(high aspect ratio) contacts, but such techniques are typicallyexpensive and cumbersome in high-volume production.

FIG. 1A (described above, see also FIGS. 1D and 1E, below) showssubstantially spherical aspect ratio solder balls 130 disposed on pads104 on a substrate 102. Each solder ball 130 has a height H which issubstantially equal to its diameter D. This is illustrative of a “1:1”aspect ratio.

FIGS. 6A-6D illustrate a process for forming high aspect ratio (tall)solder bumps, according to the invention. The process benefits from anyof the techniques for forming solder balls and bumps disclosed herein.

FIG. 6A is comparable to FIG. 1A, above, and shows a substrate 602 afterthe step of ball bumping (“bump”). Herein it can be observed that thesolder balls 630 (compare 130) are generally spherical, have a diameter“D1” (compare D, FIG. 1A) and have a height “H1” (compare H, FIG. 1A).The solder balls 630 are shown as having been formed on pads 604(compare 104) on the substrate 602 (compare 102).

For example, on a semiconductor wafer, the solder balls 630 have adiameter D1 of 5 mils and a height H1 of 4 mils, the pads 604 have asize of 4 mils×4 mils, the solder balls are substantially spherical, andthe pads (hence, the solder balls) are disposed at a pitch of 8 mils.

FIG. 6B illustrates a next step (“encapsulate”). Here, the bumpedsubstrate is encapsulated, or over molded, with a non-conductivematerial 640 such as plastic, polyimide, or silicone. This can be doneusing spinners, or potting, or dispensing. The thickness H2 of thematerial 640 preferably greater than the height H1 of the solder balls(H2>H1). For example, the thickness H2=8 mils. The top surface of thematerial 640 may be wavy, as illustrated.

FIG. 6C illustrates a next step (“lap”). Here, the over molded substrateis lapped (polished, ground) to reduce the thickness of the material640, also to planarize the surface of the material 640, and to exposethe solder balls 630. Lapping should preferably proceed until thethickness H3 of the over molding material 640 is less than the height H1of the solder balls 630. (H3<H1).

This means, of course, that the solder balls will also be lapped,resulting in their having flat top surfaces 630 a exposed. For example,the thickness H3=3.5 mils. Generally, the dimension H3 is preferably60-90% of the dimension H1, such as 70-80%, 75-90%.

It is within the scope of the invention that a selective (e.g.,chemical) etching process can be used, either during step 2 or afterstep 2, so that tops of the solder balls are either (i) recessedslightly below or (ii) extend slightly above the resulting top surfaceof the over molding material. For example, the top surfaces 630 a of thesolder balls 630 may be recessed 0.2 mils below the surface 640 a of theover molding material. Or, for example, the top surfaces 630 a of thesolder balls 630 may extend 0.2 mils above the surface 640 a of the overmolding material. Or, as shown, the top surfaces 630 of the solder ballscan be coplanar with the top surface of the over molding material 640.

It is within the scope of the invention that an alternative toovermolding and lapping and would be, after step 1 to press the balls630 against a soft rubber (or the like) substantially planar surface(not shown) that would protect the top surface of the balls 630 and actas the top mold plate to limit plastic flow when molded. (The topportions of the balls 630 would embed themselves in the rubber surface.)In this case, the material 140 would simply have a thickness less thanH1, and the top portions of the balls would extend out of the material140, thereby alleviating the need for lapping (step 3) to expose the topsurfaces of the balls.

The first three steps (bump, over mold, lap; or rubber surfacealternative) result in an interim product which is an encapsulatedelectronic component suitable for mounting directly to a PC board. It iswithin the scope of the invention that the interim product may befurther processed, as follows.

FIG. 6D illustrates a next step (“bump, again”). Here, a second set ofsolder balls 650 are formed atop the bumped/over molded/lappedsubstrate. Each second solder ball 650 is formed atop a correspondingone of the over molded/lapped solder balls 630. It is desirable whenforming the second bump (ball) 650 not to remove or to wick out thefirst bump (ball) 630, and create a large dual volume bump on thesurface. Limiting the opening at lapping is one method. Using thecaptured cell technology described herein is an effective method torestrict surface tension forces.

The resulting ball bumped substrate is a final product and can be usedwith standard printed circuit materials and methods. The additional bumpheight improves resistance to thermal and mechanical stresses. Themolding material offers ionic protection to the delicate semiconductorcircuit, and the corrosive materials used during soldering.

The resulting solder ball structure of one ball 650 atop another 630 hasa high aspect ratio. Rather than calling it a “ball” or a “bump”, it maybe termed a “reflowable interconnect structure”.

It is within the scope of the invention that the final product shown inFIG. 6D can further be processed by further over molding and lapping, inthe manner described with respect to FIGS. 6B and 6C, resulting in evengreater height for use with even smaller ball diameters and pitch.

It is within the scope of the invention:

-   -   after the step 3 (lapping), selective etching the solder balls        or the plastic encapsulating material (discussed above).    -   after the step 3 (lapping), metallize the top surface, including        fanning out.    -   after the step 3, metallize the top surface, mount de-coupling        capacitors.    -   in lieu of steps 2 and 3, embedding the top portions of the        balls in a resilient mold surface (discussed above).    -   chip scale packaging (CSP).    -   after the step 3 (lapping), mounting a flex circuit to the top        surface.    -   after the step 4 (bump again), repeating the steps 2, 3 and 4,        resulting in yet greater interconnect height.        High Volume Solder Bumps

High aspect ratio solder bumps are discussed immediately hereinabove.Generally speaking, the greater the volume of solder material in thesolder bump, the better. This is believed to be because solder willeventually initiate (start) a crack at or near the interface of the bondpad to the solder bump, when subjected to thermal cycles. This crackwill propagate a given distance per thermal cycle after it initiates.Usually as the crack propagates far enough a second crack initiatesopposite the first, and this continues across the diameter of the bump.The number of temperature cycles to crack initiation, and the rate atwitch the cracks propagate are mostly dependent on the maximum stresspresent. High (tall, high aspect ratio) bumps or greater distancesbetween substrates decreases the maximum stress present at thermalcycles and therefore increases the durability of products by increasingthe number of cycles it takes to initiate cracks, and it slows down thepropagation rate—resulting in increased useful life.

FIGS. 1C-1E (compare FIG. 1A) show substantially spherical solder balls130 disposed on pads 104 on a substrate 102. The pads 104 are typicallysquare (as shown), or octagonal (stop sign shaped). Each solder ball 130has a height H which is substantially equal to its diameter D. Since thesolder balls are spherical, they have substantially a 1:1 aspect ratio.Since the solder balls are spherical, they have a volume ofsubstantially 4/3*pi*(d/2)³.

As discussed above, the mask (e.g., 110) has a plurality ofopenings/cells (e.g., 112, 312 c), each corresponding to a pad 104whereupon it is desired to form a solder ball 130 on the substrate 102.The mask openings are typically substantially the same size and shape asthe bond pads 104.

FIGS. 7A-7C illustrates an embodiment of high volume, aspherical solderbumps 730 (compare 130) disposed on pads 704 (compare 104) on asubstrate 702 (compare 102). The term “bumps” is used in describing thisembodiment, rather than “balls”, because the bumps are not substantiallyspherical. The bond pads 704 are asymmetrical, in this example simplyrectangular, having a long dimension b1 and a short dimension h1.Aligning the long dimension b1 perpendicular to the stress produced inassemblies can improve the use full life dramatically by reducingmaximum stress present by;

-   -   1. Increasing bump height    -   2. Dissipating stress over a greater area    -   3. Dissipating stress by having one portion of the bump always        under strain (force is applied at center and outer solder        material pushed while inner material ripped, then reversed as        temperature cycle is reversed as compared to a tower of material        bent back and forth to failure.

4. Life also improved further by greatly increased crack propagationdistance as it progresses down the long axis of the bump.

High volume solder bumps 730 are formed using the techniques describedherein (for example, with respect to FIGS. 3A, 3F, 3G, 3H). In thiscase, the mask (e.g., 110) would have a plurality of openings/cells(e.g., 112, 312 c), each corresponding to a pad 704 whereupon it isdesired to form a solder bump 730 on the substrate 702. The maskopenings are typically substantially the same size and shape as the bondpads 704—in this example, rectangular.

In this case, the resulting solder bump 730 is not substantiallyspherical. It has a height H′ (which may be comparable to or greaterthan the height H of the solder bumps 130), a dimension b2 along a majoraxis aligned with the long dimension b1 of the pad 704, and a dimensionh2 along a minor axis aligned with the short dimension h1 of the pad704.

It should be noted that, although the aspect ratio for the solder bumps730 is lower than 1:1, they nevertheless benefit from having increasedmass (volume), which (roughly speaking) translates into increasedreliability.

FIGS. 7D-7F illustrate an alternate embodiment of high volume,aspherical solder bumps 760 (compare 730) disposed on pads 734 (compare704) on a substrate 732 (compare 702). The term “bumps” is used indescribing this embodiment, rather than “balls”, because the bumps arenot substantially spherical.

In this embodiment, rather than having one asymmetrical pad (704) persolder bump (730), each solder bump 760 is formed a pair of bond pads734 a and 734 b (compare 704) which are spaced apart from one another.There is a gap 736 (see FIG. 7D) between the two bond pads 734 a and 734b. The two pads 734 a and 734 b, in aggregate, form an “aggregate”asymmetrical bond pad 734 which, in this example is simply rectangular,suitably (but not necessarily) having the same long dimension b1 andshort dimension h1 as the single asymmetrical pad 704. In other words, agiven pair of pads 734 a/b suitably has the same overall profile(outline) as a given pad 704. For example, each bond pad 734 a (or 734b) is 2 mils×2 mils, and the gap between the two bond pads is 1 mil.Therefore, the resulting “aggregate bond pad” 734 is 5 mils×2 mils(b1×h1).

High volume solder bumps 730 are formed using the techniques describedherein (for example, with respect to FIGS. 3A, 3F, 3G, 3H). In thiscase, the mask (e.g., 110) would have a plurality of openings/cells(e.g., 112, 312 c), each corresponding to a pair of bond pads (an“aggregate” bond pad) 734 whereupon it is desired to form a solder bump760 on the substrate 732. To this end, the mask openings are typicallysubstantially the same size and shape as the pair of bond pads 734—inthis example, rectangular.

In this case, the resulting solder bump 760 is not substantiallyspherical. It has a height H′ (which may be comparable to or greaterthan the height H of the solder bumps 130), a long dimension b2 along amajor axis aligned with the long dimension b1 of the aggregate pad 734,and a short dimension h2 along a minor axis aligned with the shortdimension h1 of the aggregate bond pad 734.

In this example, the long dimension b2 is approximately 5/2 (250%) theshort dimension h2. It is within the scope of the invention that thedimension b2 is from 1 to 5 times greater than the dimension h2,including from 1.5 to 3 times greater, including from 1.5 to 5 timesgreater, 2 to 4 times greater, 2-5 times greater, and that it may begreater than 5 times greater.

The solder bump 760 is substantially similar to the solder bump 730,with the exception that since it is formed on a pair of two spaced-apartpads 734, it has a void space (notch, gap, recess, void) 762 (see FIG.7F) on its bottom surface, essentially in the middle of bottom surfaceof the solder bump 760 and extending transversely (minor axis) acrossthe bottom surface of the solder bump 760. This forms a bridge-likestructure (a structure supported at both ends, and not in the middle)which permits the solder bump 760 to accommodate stresses and strains,particularly in the longitudinal (major axis) direction, better thancomparable spherical solder balls (e.g., 730). This also interrupts theinitiated crack that would normally propagate to failure, this crackinterruption offers improved reliability and fault tolerant assembliesto be produced.

The aspect ratio for the solder bumps 760 is approximately 1:1 in oneaxis, and substantially is lower than 1:1 in the other axis. Althoughlower along the major axis, the solder bumps benefit from havingincreased mass (volume), which (roughly speaking) translates intoincreased reliability.

FIGS. 7G-7I illustrate an alternate embodiment of solder balls 790(compare 760) disposed on pads 764 (compare 734) on a substrate 762(compare 732). The term “balls” is used in describing this embodiment,rather than “bumps”, because the bumps are substantially spherical.

In this embodiment, rather than having one asymmetrical pad (704) persolder bump (730), or two spaced-apart pads (734 a and 734 b), the bondpad 764 is symmetrical and is formed as a ring having a diameter and ahole 766 in the middle. The bond pad 764 has an outer diameter b3 and aninner diameter b4. For example, the outer diameter b3 is 5 mils, and theinner diameter b4 is 2.5 mils.

Solder balls 760 are formed using the techniques described herein (forexample, with respect to FIGS. 3A, 3F, 3G, 3H). In this case, the mask(e.g., 110) would have a plurality of openings/cells (e.g., 112, 312 c),each corresponding to a bond pad 764. whereupon it is desired to form asolder bump 790 on the substrate 762. To this end, the mask openings aretypically substantially the same size and shape as the bond pads 764—inthis example, round (see, e.g., FIG. 3B), although rectangular (orsquare) openings can contain more solder paste for a givencross-dimension.

In this case, the resulting solder ball 790 is substantially spherical.It has a height H″ (which may be comparable to or greater than theheight H of the solder bumps 130), and a diameter b5. In this example,the diameter b5 is substantially equal to the height H″, resulting in anaspect ratio of substantially 1:1.

The solder bump 790 is substantially similar to the solder bump 730 (or130), with the exception that since it is formed on a ring-shaped pad764, it has a void space (notch, gap, recess, void) 792 (see FIG. 7I) onits bottom surface, essentially in the center of bottom surface of thesolder ball 790, on the bottom surface of the solder ball. This gap 792serves the same purpose as the gap 762—namely, accommodating stressesand strains, but in this case symmetrically in both axes.

It is within the scope of the invention that the gap 736 or the hole 766is filled with a dollop of material such as plastic, polyimide, orsilicone to prevent formation of a solder bump 760 or solder ball 790within the gap.

Assembling the Mask to the Substrate

FIG. 8A illustrates an alternate embodiment of a an “assembly” 800 of amask 802 (compare 110) and a substrate 804 (compare 102, The substrate804 is supported by a workholder (stage, chuck) 806, which is on themachine platform 810. pedestals 812 and 814 extend upward (towards themask) from the workholder 806. The mask 802 is shown having a mask mount816 fixing a one edge thereof. When the mask 802 is brought down ontothe substrate 804, a vacuum is drawn through the vacuum pedestals 812and 814 to hold the mask 802 intimately against the substrate 804.

FIG. 8B illustrates how a pressure plate 820 (compare 410) may be addedto the “assembly” (800) of mask 802 and substrate 804. The assembly 800is inverted and disposed onto the pressure plate 820. The pressure platemay simply be a stainless steel plate which is held by pedestals 822extending upwards (towards the pressure plate) from the machine platform810. A heater stage 824 (compare 412) is disposed underneath thepressure plate. If desired, the pressure plate 820 may be secured to theassembly 800, in intimate contact with the mask 802 using magnets,vacuum chucks and the like.

It is within the scope of the invention that any combination of gizmos,gadgets, and the like (cam surfaces, vacuum chucks, magnets,electromagnets) can advantageously be utilized to hold the mask to thesubstrate and to hold the pressure plate to the mask.

Preferably, as shown and described with respect to FIG. 10, a set oflift magnets (1028) hold the mask (via carrier 1020) to the chuck base(1014). Generally, magnets are preferred over vacuum.

Preferably, as shown and described with respect to FIG. 10D, the mask(1010) is glued in a stainless steel (SS) mesh (1013) in a frame (1012).

Biased Chuck

As mentioned above, a mask is placed substantially into face-to-facecontact with a substrate being bumped. When the assembly of the mask andthe substrate are moved (re-positioned), such as to an inverted orsemi-inverted position, the mask may separate somewhat from thesubstrate, allowing solder material to enter gaps between the mask andthe substrate. Also, during reflow, the mask may warp or buckle, alsoallowing solder material to enter gaps between the mask and thesubstrate. According to an aspect of the invention, a biased chuckassembly is provided for maintaining an intimate face-to-face contactbetween a mask and a substrate being bumped.

FIG. 9 illustrates a biased chuck assembly 900 for holding a substrate902 such as, but not limited to, a semiconductor wafer in positivecontact with a mask 904. In a manner such as described hereinabove, twoopposite edges of the mask 904 may be retained by rails 906 and 908, sothat the mask 904 can be tensioned (stretched). See FIG. 10D for a morepreferred technique for tensioning the mask.

Semiconductor wafers are relatively brittle, but are known to have acertain degree of flexibility. For purposes of practicing thisinvention, the degree of flexibility possessed by a semiconductor waferis sufficient to allow the semiconductor wafer 902 to deflect when urgedagainst the mask 904 so as to maintain substantially intimate contactbetween the surface of the mask 904 and the surface of the semiconductorwafer (substrate) 902.

The substrate 902 is urged against the mask 904 in the following manner.A rigid, generally planar chuck base 910 has a central recess (cavity)912 extending into the chuck base 910 from a top (as viewed) surfacethereof. The recess 912 is sized and shaped to receive a generallyplanar, flexible diaphragm 914. The diaphragm 914 extends across therecess 912, and is secured to the chuck base 910 such as with a bead 916of a suitable adhesive 916 disposed about the periphery of the diaphragm914. An inlet tube 920 extends from exterior the chuck base 910 towithin the cavity 912, underneath the diaphragm 914. In this manner,when a gas such as nitrogen is introduced at a positive pressure intothe inlet tube 920, the diaphragm 914 is caused to deflect upwards (asviewed), urging anything disposed atop the diaphragm 914 (in this case,the wafer 902) upwards (in this case, against the mask 904). Thediaphragm 914 is suitably a 0.125 inch thick sheet of silicon rubbermaterial. The peripheral edge of the diaphragm 914 is preferably“contained” by the side wall of the cavity 912, as illustrated.

Preferably, a permeable substrate 928, such as a 100 mil thick powderedmetal plate, is disposed beneath the diaphragm 914, between thediaphragm 914 and the bottom surface of the cavity 912. When a suctionis applied to the inlet tube 920, the permeable substrate 928 willprevent the diaphragm 914 from closing off the opening.

A second central recess (cavity) 922, coaxial with and larger (wider, ofgreater diameter) than the recess 912 extends into the chuck base 910from the top surface thereof, and is sized and shaped to receive agenerally planar, flexible manifold element 930.

As best viewed in FIG. 9A, the manifold element 930 has a top surface932 and a bottom surface 934. A plurality of grooves 936 extend, such ascrisscross style (2 parallel sets of intersecting grooves), across thetop (as viewed) surface of the manifold element 930. An opening 938extends from the top surface 932 of the manifold element 930 (or from abottom of one of the grooves 936) through to the bottom surface 934 ofthe manifold element 930. The opening 938 is aligned with an inletorifice 940 in the chuck base 910.

As best viewed in FIG. 9, the manifold element 930 extends across therecess 922, and may be secured to the top (as viewed) surface of thediaphragm 914. (Alternatively, the manifold element 930 may be formedintegrally with the diaphragm.) In this manner, when a vacuum is pulledon the inlet tube 940, a substrate 902 sitting atop the manifold element930 is held in intimate contact with the manifold element 930. Themanifold element 930 is suitably a 5 mil thick sheet of a film materialsuch as Kapton™.

In use, a wafer 902 is loaded onto the chuck assembly 900. The wafer 902is disposed atop the manifold element 930. The mask 904, which maypreviously have had solder material introduced into its cells(apertures), is disposed against (including nearly against) the surfaceof the wafer. A positive pressure is introduced into the inlet tube 920,and the assembly of mask and wafer can be manipulated (e.g., inverted,partially-inverted) for reflowing the solder material, as discussedhereinabove. Intimate contact is assured between the mask and thesubstrate by the positive pressure at the inlet tube 920. After thesolder material has been reflowed, preferably after the solder ballshave formed on the substrate, a negative pressure (vacuum) is applied toboth of the inlet tubes 920 and 940 to hold the wafer 902 firmly to thechuck assembly 900 so that the mask 904 may be lifted off of (releasedfrom) the wafer 902.

An additional advantage of the chuck assembly 900 is that the wafer 902is disposed upon a non-metallic film 930 which, in turn, is disposedupon a non-metallic membrane 914, both of which (930 and 914) serve asthermal barriers to isolate the thermal mass of the chuck base 910 fromthe wafer 902. Inasmuch as it is generally preferred to keep the thermalmass “seen” by the heater element to a minimum so that the soldermaterial in the mask may efficiently be reflowed, this serves to reducethe effective thermal mass of the chuck assembly. This also evens theload across great areas, without the normal high and low pressures seenusing rigid chucks.

Examples of Solder Materials and Mask Dimensions

A suitable solder material for use with the present invention comprises“63/37” lead/tin solder having a melting temperature of approximately183° C. (Centigrade), and has relatively large particle sizes. Largesolder particles are less likely to leak out of any gap (e.g., 314)between the mask and the substrate being bumped. The following chartlists a number of exemplary dimensions and relationships between:

-   -   D, the diameter of the desired resulting solder ball;    -   W, the cross-dimension of the cell in the mask;    -   T, the thickness of the mask;    -   d, the particle size (e.g., diameter);    -   #, the approximate number of particles in a cell; and    -   %, the final percentage of metal, by volume, in the cell.

D (mils) W (mils) T (mils) d (mils) # % 4  6 3 1.5 18 31 5 7-8 4 2 15 2810 12-13 8 4 37 42 20 25 15 5 63 44

Notes:

-   -   1. The pitch of the pads on the substrate being bumped is        typically twice the diameter (D) of the resulting solder ball.    -   2. The size of a pad on the substrate being bumped is typically        approximately equal to the diameter (D) of the resulting solder        ball.    -   3. The final percentage (%) metal is determined without        compression of solder material in the cell.

From the chart presented above, it is evident that:

The cross-dimension (W) of a mask cell is always greater than thethickness (T) of the mask.

The solder material filling the cells in the mask preferably comprisessolder particles which of a size (d) which is relatively “huge” incomparison to the cell cross dimension (W) or diameter (D) of theresulting solder ball. As is evident from the chart presented above, thedimension “d” is at least approximately 20% of the dimension “W”. And,the dimension “d” is at least approximately 25% of the diameter “D” ofthe resulting solder ball.

According to the invention, the solder material comprises solderparticles of a size (d) which is at least 10% of either thecross-dimension (W) of the mask cell or the diameter (D) of theresulting solder ball, including at least 20% of the cross-dimension (W)of the mask cell or which is at least 25% of the diameter (D) of theresulting solder ball. As compared to mask thickness (T), the smallestparticle diameter (d) should be at least 40% of the mask thickness,including at least 50%.

An advantage of using “huge” solder particles in the solder material isthat the particles will be less likely to “leak out” of any gap (e.g.,314) between the mask and the substrate. A typical dimension for a gapbetween a mask and a substrate being ball-bumped, due to non-planarity'sin the substrate, may be on the order of 1-2 mils.

Another advantage of using “huge” solder particles is volume control,and increasing the percentage of solid material in each cell of themask, so as to maximize resulting solder ball size. Using a typicalsolder paste, which is a homogeneous suspension of metal powder in aflux vehicle, the percent solid material is limited by the solder pastecomposition. In contrast thereto, huge particles, when forced into thecell, will displace flux, and may also compact (deform). In this manner,a surprisingly large volume percentage can be achieved.

It should also be understood that the solder particles in the soldermaterial used to fill the cells in the masks of the present inventionare not necessarily spherical, in which case they would have a width orcross-dimension rather than a “diameter”.

In the context of there being gaps between a mask carrying the soldermaterial and a surface of the substrate being ball-bumped, the solidparticles preferably exhibit a minimum diameter which is larger than thelargest gap between the mask and the substrate.

A suitable solder material contains particles of lead/tin solder, in amatrix of flux, with the following proportions: 80% (by weight) solidmaterial (e.g., particles of lead/tin solder), and 20% (by weight) flux(including volatiles). In terms of relative volume percentages, the sametypical solder material may contain approximately 55% (by volume) ofsolid material (metal) and 45% (by volume) of flux.

According to the invention, a suitable solder material for use in beingapplied to a substrate and reflowed to form solder balls on thesubstrate has the following composition and characteristics:

-   -   a plurality of solid particles of solder material suspended in a        flux-material;    -   the solid particles having diameters in the range of from        approximately 1.5 mils to approximately 5.0 mils.

Preferably, the average size of the solder particles is such that theynumber (#) in the range of a few dozen to a few hundred solder particlesfilling each cell of the mask.

Novelty and Non-Obviousness

The present invention includes many features which are not taught orsuggested by the prior art, including but not limited to the one or moreof the following features taken either alone or in combination with oneanother:

-   -   captured cell;    -   biased chuck;    -   square mask openings;    -   off-line (away from the wafer) filling of the mask;    -   the use of huge solder particles, and compaction;    -   ensuring a gap (non-contact ball bumping) between the mask and        the substrate; heating via the mask rather than through the        substrate;    -   reflowing partially inverted; and    -   un-inverting before cooling.

For example, the inverted reflow feature of the present invention isdistinguishable over that which was described in the IBM-2 patent. TheIBM-2 patent fails to use a captured cell. It is believed that the IBM-2process, lacking the captured cell feature of the present invention,would result in molten solder leaking out of the fixture.

For example, the use of such “huge” solder particles is a non-obviousdeviation from the use of solder pastes as indicated by theaforementioned Hewlett Packard, IBM-1 and IBM-2 patents.

As mentioned in the Hewlett Packard patent, solder paste is ahomogeneous, stable suspension of metal powder in a flux vehicle. Thelargest allowed particle diameter should be below 40% of the maskthickness. As mentioned above, according to the present invention, thesmallest particle diameter (d) should be at least 40% of the maskthickness, including at least 50%.

For example, the present invention is in marked contrast to any priorart that significantly heats the substrate being bumped, or that heatsthrough the substrate being bumped. The substrate provides an unreliableconductive path for heat, and imposing thermal stresses upon thesubstrate is generally undesirable. It is thus preferred, as disclosedherein, to direct heat at the mask so reflow the solder material in thecells of the mask.

Another advantage of the present invention is, as described hereinabove,since the solder ball has a diameter which exceeds the thickness of themask and sticks out when reflowed, it can join itself to the substratewithout there having been any contact between the mask and thesubstrate. Also, the mask can be removed while the solder is stillmolten, thereby greatly facilitating mask/substrate separation.

Although the invention has been illustrated and described in detail inthe drawings and foregoing description, the same is to be considered asillustrative and not restrictive in character—it being understood thatonly preferred embodiments have been shown and described, and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. Undoubtedly, many other “variations” on the“themes” set forth hereinabove will occur to one having ordinary skillin the art to which the present invention most nearly pertains, and suchvariations are intended to be within the scope of the invention, asdisclosed herein.

For example, the heater stage could be left in place while the solderballs cooled off and solidified (i.e., rather than separating the heaterstage from the assembly of the mask and the substrate), in which casethe resulting solder balls would have flat tops. However, in light ofthe desire to reutilize the preheated heater stage as quickly aspossible, without needing to bring it back up to temperature, such ascheme is generally not preferred.

For example, although the invention has been described mainly in termsof the mask being in face-to-face contact with the substrate beingbumped, it is within the scope of the invention that a small (e.g.,0.25-0.75 mil) spacing is maintained between the mask and the surface ofthe substrate to prevent any damage to a delicate substrate surface thatmay result from contact with the mask. Since the method of the presentinvention handles gaps resulting from substrate surface topology, it isevident that maintaining an overall gap between the mask and thesubstrate is feasible.

For example, although a solder material comprising solder particles andflux is described, the solder material may be dry, such asfluorine-treated, or using a forming or reducing gas.

For example, any suitable heating profile may be used to reflow thesolder material, such as in accordance with the manufacturer'sspecifications.

For example, the mask may be coated with a polymer such asphoto-imageable polyimide or silicone rubber. This will protect thesubstrate against damage if the mask is in contact with the substrate.The coating, if sufficiently thick, can also serve as a conformal maskmating to irregular surfaces, and improve the volume of solder per cell,and help release the substrate.

For example, after ball-bumping one substrate (or a plurality ofsubstrates in a workholder, in preparation for the ball-bumping the nextsubstrate (or the next batch of substrates) the mask is preferablycooled, for example by blowing nitrogen gas over it, to get it below theactivation temperature of the flux (which is lower than the meltingpoint for the solder). For example, to cool the mask off toapproximately 50° C., or lower.

Many of the features discussed hereinabove can be “mixed and matched”with one another. Other features are generally incompatible with oneanother—for example, it might be inapposite to have a biased chuck as inFIG. 9 along with a bridging the gap embodiment as in FIG. 5A. Onehaving ordinary skill in the art to which the invention most nearlypertains will understand which features work well with one another andwhich do not.

In the main hereinafter, substrates which are semiconductor wafers(“wafers”) are discussed, but the invention is not limited to wafersubstrates.

Captured Cell

One of the distinguishing, and rather critical features of the inventionover many of the prior art approaches is that the present invention uses“captured cell” technology. As described above, the cells can be closedby a pressure plate (e.g., 120, 170, 410, 472, 564, 820) by the heaterstage itself (e.g., 230, 280, 330) or by using a mask (e.g., 500) withblind holes (512).

In the embodiments described hereinbelow, the cells of the mask aretypically closed off by the heater stage itself, without a separatepressure plate.

Characteristics of the Mask

The mask should have low thermal expansion, with holes which are etchedrather than drilled. This is applicable to masks that have cells whichare either through holes, or which are blind holes.

Mounting the Mask

It was previously believed that permitting the mask to expand freely, inone axis, would be the best way to alleviate problems associated withwarpage (warping), and this is suggested in FIG. 8A (mask 802 mounted atone edge by 816).

A more preferred system for mounting the mask has been developed. Themask is, for example, a molybdenum sheet with holes. The mask ispreferably mounted to a stainless steel (SS) mesh (screen) which ispre-tensioned on a disposable frame. The mask and SS mesh are gluedtogether. Then the SS mesh is cut away from the center of the mask (thisapplies tension to the mask), where the holes (cells) are. One edge ofthe mask is directly attached. The opposite edge has approximately oneinch (2.54 cm) of SS mesh between the mask and the frame. This allowsthe mask to expand, and the SS mesh takes up the slack. This also allowsthe frame to change temperature without affecting mask tension. If allfour sides of the mask were directly mounted to the frame, as the framecooled, the mask could buckle (or “oil can”). See FIG. 10D, describedbelow.

Reducing Forces

In the embodiments described hereinbelow, printing (filling the maskcells) is mainly done “off-line”, without the mask first being on thewafer. This is important in that it reduces the force required on thewafer from print blade forces, and reduces cell volume variations, asdescribed in greater detail hereinbelow.

Capturing the Cells

The cells are closed by the heater stage itself. Magnets are disposedabout the periphery of (located outside of) the mask frame and theheater stage to hold the heater stage to the mask frame. Also, afterfilling the cells with solder paste, the opposite side of the mask isclosed by the wafer (as an example of a substrate being ball bumped).Magnets are disposed about the periphery of (located outside of) thechuck assembly to hold the chuck assembly to the mask frame. The maskframe slides into the carriage with land areas for magnets to contact.The carriage moves the mask and frame assembly to the chuck, etc. Inthis manner the force required on the wafer to maintain the capturedcell can substantially be reduced, which has been found to bebeneficial.

An Exemplary Machine and Process Flow

FIG. 10 illustrates an exemplary ball bumping machine 1000 having a base1002, a chuck 1004 on the left side for holding a wafer 1006 and aheater stage 1008 on the right side. A mask 1010 is held in a frame1012. The chuck 1004 is disposed in chuck base 1014. The heater stage1008 is disposed in a heater stage base 1016.

An elongate shuttle (carriage) mechanism 1018 is pivotally attached tothe base 1002 at a point “P” between the chuck 1004 and the heater stage1008. The frame 1012 is held in a carrier 1020 which is attached to theopposite (free) end of the shuttle mechanism 1018. A motor 1021 controlsthe position of the shuttle mechanism 1018. The shuttle mechanism 1018can shuttle the mask 1010 (i.e., the carrier 1020) between the heaterstage 1008 on the right side (as shown) and the chuck 1004 on the leftside. The shuttle mechanism 1016 pivots about the point “P”. Cameras(not shown) are used to make alignments, for example of the mask 1010 tothe wafer 1006.

A set of holddown magnets 1022, which preferably are electromagnets,selectively hold the chuck base 1014 to the machine base 1002.Similarly, a set of holddown magnets 1024, which preferably areelectromagnets, selectively hold the heater stage base 1016 to themachine base 1002. The carrier 1020 is ferrous, or has ferrous “lands”.A set of lift magnets 1026, which preferably are electromagnets,selectively hold the carrier 1020 to the heater stage base 1016.Similarly, a set of lift magnets 1028, which preferably areelectromagnets, selectively hold the carrier 1020 to the chuck base1014.

In this manner, the mask can be brought down onto the heater stage, themagnets 1026 turned on, the magnets 1024 turned off, and the heaterstage can be lifted by the shuttle mechanism 1016. In other words, whenthe mask is shuttled, it can take the heater stage with it. Similarly,the mask can be brought down onto the chuck, the magnets 1028 turned on,the magnets 1022 turned off, and the chuck can be lifted by the shuttlemechanism 1016.

A more detailed example of mask mounting is shown in FIG. 10D where onecan see the mask 1010 glued (mounted with an adhesive 1011) to a SS mesh1013 in a frame 1012, as described hereinabove, and an area 1032 ofcells 1034, as described hereinbelow.

FIG. 10B illustrates an embodiment of a chuck assembly, according to thepresent invention in somewhat more detail than it was illustrated inFIG. 10. This is similar to the chuck of FIG. 9, but without someelements and with the addition of other elements. But the basic idea isthe same—namely, to hold the wafer and be able to introduce pressure toflex it into intimate contact with (in this example) the printed mask.

What was shown as chuck assembly 1014 in FIG. 10 can be seen to comprisean inner chuck base 1054 and an outer chuck base 1056. The outer chuckbase 1056 sits atop the machine base 1002. The lift magnets 1058(compare 1028) are located in the outer chuck base 1056. A wafer 1006 isshown disposed above everything, merely for illustrative perspective.

The inner chuck base 1054 is mounted on a set of legs 1062 within theouter chuck base 1056, and the legs allow the inner chuck 1054, hencethe wafer 1006, to be raised or lowered by a stepper motor or othersuitable actuator (not shown), as discussed above. An air cylinder 1064provides pressure for flexing the wafer, as described hereinabove.

A vacuum line 1066 extends through various (three shown) insulatinglayers 1068 (three shown) to a manifold element 1070, for holding thewafer. The manifold element is suitably mica ceramic.

FIGS. 11A-11D illustrate an exemplary process flow, as describedhereinafter. Various alignment steps are omitted from the description,as they will be well understood by the person of ordinary skill in theart to which the invention most nearly pertains.

In a first process step (FIG. 11A), the mask 1010 is disposed on theheater stage 1008, and is secured (assembled) to the heater stage byturning on the lift magnets 1026. This is before the cells of the maskare filled with solder paste, and before the heater stage is heated up.

As best viewed in FIG. 10A, the mask 1010 has an area 1032 (typicallycentrally located on the mask) that has cells 1034 extending completelythere through. A groove 1030 is formed in the top surface of the heaterstage, and preferably extends entirely around an area corresponding tothe area 1032 of cells 1034. The groove 1030 communicates with anorifice 1036 which extends to (beyond) an outer surface of the heaterstage. When vacuum is applied to the orifice, the mask 1010 is heldfirmly onto the heater stage 1008. The groove 1030 is preferably atleast one inch (2.5 cm) away from (outside of) the area 1032 of cells1034. It is preferred not to have the vacuum groove too close to thearea of the cells so as to avoid the vacuum applied thereto exerting asuction on the molten solder paste (including flux) when the heaterstage is heated up (as described hereinbelow). Since the heater stage isfunctioning as the pressure plate in capturing (closing off) the cells,it is important to maintain intimate contact with a mask having cellswhich are through holes and the surface of the heater stage, and tosubstantially prevent the mask from warping. The vacuum groove 1030achieves this purpose, while allowing for some expansion and/orcontraction of the mask 1010 without buckling.

Generally, blind hole masks (e.g., 500) are not preferred, it havingbeen found that to manufacture a blind hole mask is difficult withrespect to maintaining uniform hole depth (hence, cell volume),particularly when etching is the preferred hole-making process (in favorof drilling). The vacuum groove 1030 in the heater stage makes athrough-hole mask behave like a blind hole mask, in the sense thatleakage between the mask and the heater stage (in the role of closingoff the cells) is substantially eliminated.

The magnets 1026 “assemble” the heater stage to the mask carrier so thatthe heater stage can shuttled along with the mask. The vacuum holds themask to the heater stage, thereby capturing the cells on one side of themask. These two features have the following benefits:

-   -   keeps solder paste from leaking under through-hole type mask    -   holds mask to ensure uniform heating or outer mask area    -   holds mask during extraction, keeps mask from warping

In the case of a mask with cells which extend through the mask (asillustrated, and as preferred), any leakage between the mask and theheater stage will adversely affect the subsequent ball formation. Apressure plate may optionally be disposed between the heater stage andthe mask, but is not necessary. With a blind hole type mask, the holeswould be disposed away from the surface of the heater stage (e.g., the“pressure plate portion” 520 of the blind hole mask 500 would be againstthe surface of the heater stage), and leakage between the mask and theheater stage would not be an issue, but it is nevertheless important tomaintain intimate contact between the heater stage and the mask. Themask is relatively thin (e.g., 0.003 inches=3 mils), and is thereforeprone to warping, particularly when heated and constrained by a frame.The heater stage is relatively thick, and (in relative terms) not proneto warping. It is important in any case to maintain intimatesurface-to-surface contact between the heater stage and the mask duringnot only the mask printing step (discussed hereinbelow), but throughoutthe entire process of forming solder balls, to avoid mask warping.Maintaining mask flatness (i.e., avoiding mask warping) is veryimportant to successful ball formation and yield (e.g., avoidance ofvoids).

Meanwhile, as shown in FIG. 11A, the wafer 1006 is loaded onto the chuck1004, which is movable in the vertical axis, and the chuck may bepositioned slightly (e.g., 0.005 inches, 5 mils) below “contact height”(see, e.g., the dashed line in FIG. 10). Contact height is the height atwhich the mask will contact the wafer, when shuttled over to the left,and it is simply a good idea to leave a small clearance between the maskand the wafer so that the mask can be positioned onto the wafer withoutmechanical interference. This step can be before, during or after thefirst process step of securing the mask to the heater stage.

About the “clearance”, which is comparable to the “gap” describedhereinabove. The clearance dimension of 5 mils is about 5 times as greatas the average size of a typical 1 mil diameter solder particle fillinga mask cell. The typical mask cell has a cross-dimension (diameter, inthe case of a cylindrical cell) of approximately 4-10 mil.

Next, the mask is printed—in other words, the cells 1034 of the mask1010 are filled with solder paste (not shown, see, e.g., FIG. 1B). Thiscan be done in any suitable manner, so long as the cells of the mask aresubstantially and uniformly filled, and that there is substantially noexcess solder paste on the surface of the mask. An exemplary techniquefor printing the mask is described hereinbelow, with respect to FIG. 12.

It is preferred to print “off-line”—in other words, not on the wafer. Ifprinting on the wafer (as described in the parent application), it mustbe appreciated that the surface of the wafer is often not very flat,topographically speaking. And this topography can lead to variations inthe effective overall volume of a cell being filled with solder paste.As a general proposition, any variations in the process, fromcell-to-cell, are simply not desirable. Hence, printing on a known flatsurface—i.e., the surface of the heater stage—is preferred. Also, byprinting “off-line”, the wafer is spared from the sometimes excessiveforces required to get a good print (effective cell filling).

The heater stage is, of course, at this point in the process,substantially at “room temperature” (not heated). Else, flux in thesolder paste in the cells of the mask would start to vaporize, etc. Thisrepresents a departure from many of the processes generally described inthe parent application, where it was described to be desirable to havethe heater stage preheated, at all times.

Next, as shown in FIG. 11B, the assembly of the printed mask and theheater stage (“mask/stage”, as held together by magnets 1026) areshuttled over to the wafer 1006 which is sitting on the chuck 1004.

Then, the lift magnets 1028 are turned on firmly secure (“assemble”) themask carrier to the chuck. This ensures that the chuck and wafer padswill maintain alignment to the mask holes during transfer. Then chuckcan then be shuttled to the 135-degree (90+45 degree) position forreflowing the solder paste. (The 135 degree position is shown in FIG.10C.)

The heater stage lift magnets 1026 and the chuck lift magnets 1028 arephased (poled) oppositely so that they do not cancel out when everything(heater stage—mask—chuck) is assembled together.

Although this step of contacting the wafer to the mask is shown with thewafer in the non-inverted position, it is within the scope of theinvention that the wafer and chuck could be shuttled over to themask/stage, or that both the wafer/chuck and mask/stage could beshuttled to an intermediate position.

Next, the chuck is pressurized—for example to approximately 3 psi. Asdescribed hereinabove, this will ensure positive intimate contact of thewafer with the mask. (This will also take up the 0.005″ clearance,mentioned above.) This intimate contact is beneficial because:

-   -   it contains the solder paste during heat up;    -   keeps the mask from warping; and    -   ensures contact of the wafer pads with the balls which will be        formed in the mask.

Next, the heater stage is heated up, according to a desired profile(temperature schedule). For example, the heater stage is first heated toapproximately 150 degrees (C), which will activate the flux.

With the flux activated, the assembly of chuck/wafer/mask/stage may beshuttled to t nearly inverted position, such as 135 degrees (FIGS. 10C,11C; compare FIG. 4D). This is advantageous for wafers having irregulartopography, but is not necessary for wafers having relatively flat topsurfaces.

Next, the temperature is increased sufficiently to reflow the solderpaste and permit balls to form. For conventional 63/37 Pb/Sn eutectic,this is at least about 183 degrees (C). The preferred temperature forthe described process is 195-200 degrees.

As shown in FIG. 10C, when the solder paste reflows, it forms balls 1040(compare 340) on pads 1044 (compare 304) of the substrate (wafer) 1006(compare 302). The balls extend (grow) out of the cells 1024 of the mask1006 and wet themselves to the pads of the wafer. This semi-invertedorientation causes solder paste to be forced (by gravity) into a 90degree corner (bottom left, as viewed) in each cell of the mask,allowing venting at the opposite corner (top right, as viewed).

Finally, the wafer is extracted after some time (dwell) at maximum(solder reflow) temperature. The pressure (e.g., 3 psi) is turned off atthe chuck, and the wafer is slowly pulled away from the mask. This isadvantageously done before the re-flowed solder material has solidified,thereby facilitating mask removal. However, caution should be exercisedwith respect to slowly separating the mask from the wafer (or viceversa) to that air currents and/or suction are not created. For example,a separation speed of about 2 inches per second has been foundreasonable.

A chamber is optionally formed between the chuck and the mask holder sothat the atmosphere can be controlled, e.g., NO₂.

After mask removal, the heater stage can be shuttled back (with the maskholder) to its original position, awaiting the next cycle.

Then, the mask can be moved to a neutral position for removal orcleaning.

Observations

The molten solder ball remains in contact with the mask edges inside theaperture (cell), depending on the amount of interference. For example, a0.004 ball inside a 0.003 mask will have 0.001 interference. The ballflat will be located at some distance from the aperture wall at anyrotational position. That means if the pad were to be skewed to one sideof the aperture and the ball on the other side, a “miss” could occur (nocopper pad in contact with liquid solder). Therefore, good initialalignment is very important.

Normally, the balls are formed on the wafer with the waferuninverted—with the pads atop the wafer (rather than below, or“inverted”, as discussed in detail in the parent application). The 135degree partially-inverted scenario (FIG. 10C) appears to only berequired with wafers having high topography (deviating from flat). Otherangles, such as between 20 or 30 degrees and 60 degrees are believed tobe beneficial for partially-inverted. With highly planar (lowtopography) wafers, however, little difference is observed betweenreflowing partially-inverted and non-inverted.

During reflow, solder paste (particles of solder in flux) first outgasessome flux, then the solder balls begin to shrink into a slug (nointerference is observed when solder is a slug). Then a complete meltingand complete surface tension equilibrium causes interference and theliquid solder wets to the solid copper pad. This is the reason forhaving little to no voids in the solder pad interface. In anexperimental bumping situation, only 0.4% of the pads had voids, and thevoids were less than 5% of pad diameter.

A significant benefit accrues to printing the mask without the waferbeing present. Normal print pressure is on the order of 60 psi (poundsper square inch), and this is a lot of pressure to subject a wafer to.By avoiding this, the only pressure exerted on the wafer is the 3 psiused to flex the wafer into intimate contact with the mask prior toreflow.

Printing (Filling the Mask Cells)

FIG. 1 (above) illustrates a technique 100 for forming solder balls on asurface of a substrate 102, such as is set forth in U.S. Pat. No.5,988,487. The substrate 102 has number of pads 104 on its top (asviewed) surface. The pads 104 are typically arranged in an array, havinga pitch (center-to-center spacing from one another). The substrate 102is disposed atop a heater stage 106. A mask (stencil) 110 is provided.The mask 110 is a thin planar sheet of relatively stiff material, suchas molybdenum, having a plurality of openings (cells) 112, eachcorresponding to a pad 104 whereupon it is desired to form a solder ballon the substrate 102. The mask 110 is placed on the top (as viewed)surface of the substrate 102 with the cells 112 aligned over the pads104. The cells 112 in the mask 110 are filled with solder material 114.This is done in any suitable manner such as by smearing solder materialon the top (as viewed) surface of the mask 110 and squeegeeing thesolder material 114 into the cells 112 of the mask 110. Squeegeeing istypically a multi-pass process.

The cells 112 in the mask 110 may be filled with solder paste prior toplacing the mask 110 on the top surface of the substrate, in which casethe solder-paste-filled cells 112 would be aligned over the pads 104.

A pressure plate 120 is disposed onto the top (as viewed) surface of themask 110. This holds the mask 110 down onto the substrate 102, and thesubstrate 102 down onto the heater stage 106. This also closes off thecells 112 (“captured cell”). The heater stage 106 is heated up,typically gradually, to a temperature sufficient to cause the soldermaterial in the cells 112 to melt (reflow). When the solder materialmelts, the individual solder particles will merge (flow) together and,due to surface tension, will try to form (and, typically, will form) asphere. When the solder material re-solidifies, it assumes a generalspherical or hemispherical shape. The mask 110 is then removed from thesubstrate 102.

FIG. 1A (above) is an enlarged (magnified) view of the substrate 102shown in FIG. 1, after completion of ball bumping. Herein it can beobserved that the solder balls 130 are generally spherical, have adiameter “D” and have a height “H”.

When printing, for example, on the surface of an integrated circuitwafer, it must be appreciated that the surface of the wafer is often notvery flat, topologically speaking. And this irregular topology can leadto variations in the effective overall volume of a cell being filledwith solder paste. Also, as mentioned above, when printing on anirregular surface, solder paste can ooze out under the mask, creatingsubsequent problems during reflow. As a general proposition, anyvariations in the process, from cell-to-cell, are simply not desirable.Hence, printing on a known flat surface (KFS)— such as the surface ofthe heater stage (e.g., 106)—is preferred. Also, by printing “off-line”,the wafer is spared from the sometimes excessive forces required to geta good print (effective cell filling). we didn't mention the excessiveforce problem above.

According to an aspect of the invention, it is generally preferred toprint “off-line”—in other words, with the mask on a smooth surfacewithout irregularities, rather than on the surface of an electroniccomponent (e.g., substrate 102). This is for purposes of (i) uniformityand (ii) to avoid damaging an underlying component.

Off-wafer printing is good for three reasons:

-   -   1: Low force    -   2. Excellent cell volume control    -   3. Finished solder void control—when the solder paste wets to        the pad of a part to be bumped the flux can be trapped during        the reflow (solder voids). With off wafer printing the solder        paste is not wetted to the pad, the ball is sphere-ized in the        mask and only contacts the pad after liquefied this avoids flux        trapped voids. No other process offers this void avoidance.

Printing off-line is illustrated, for example, in FIG. 4 of theaforementioned Parent Application which is a schematic diagram of amachine for ball bumping substrates including a print station 414, whichmay be a flat, non-wettable surface for off-wafer filling of the cellsof the mask with solder material.

The flat surface is non-wettable from the solder material's perspective.Suitable materials are Teflon™ coatings and chrome. The flat surfaceshould not only be free from surface topology and defects such asscratches or dings and dents, but will remain flat during heating athigh rates. Heat differences coupled with the materials expansionproperties may result in warpage during heating.

FIG. 12 illustrates an embodiment of the mask-filling technique of thepresent invention. It should be understood that the technique is notlimited to filling masks for the purpose of ball bumping electroniccomponents, and has more general applicability to any number of printing(mask filling) processes, whether ball bumping or otherwise. It shouldtherefore also be understood that the present invention is not limitedto filling masks with any particular solder paste or, for that matter,with solder paste at all. The technique is well-suited to filling thecells of the mask with any material having a viscosity in the range of20 kcps-300 kcps (kilocentipoise).

As shown in the figure, a quantity (blob, glob, mass) of solder paste1202 is disposed on the surface of the mask 1210 (compare 110). The mask1210 is shown as being disposed on a suitable support surface 1208(compare 106, or 414 of Parent Application). The support surface 1208may be a wafer, for printing with the mask 1210 already disposed on awafer (compare 102), if so desired. Or, the support surface 1208 may beany non-wettable surface for off-line filling of the mask.

The mask 1210 has a plurality of cells 1234 (compare 112) which may bearranged in an array. The cells 1234 may be round, square or the like.The mask has a thickness, typically 3 mils. The cells are preferably,but not necessarily, uniform in size, hence volume. For example, asquare cell may have a cross-dimension of 6 mils.

A first “print” (or “flood”) blade 1220, such as a rubber blade made of90 Durometer ULON™, is brought to a distance of a few mils (e.g., 5-7mils) from (above) the surface of the mask 1210. The blade 1220 isadvanced in the direction of the arrow 1222. As the blade 1220 advances,the cells 1234 become filled with solder paste 1202 (compare 114). It ispreferred that the blade 1220 not contact the mask, and not drag acrossthe mask. Because the blade 1220 is spaced from the mask 1210, therewill inevitably be an amount of excess solder paste on the surface ofthe mask behind (to the left of, as illustrated) the blade 1220.

Since the blade 1220 is not in contact with the mask 1210, the contactpressure is essentially zero. This can be important when the mask 1210is supported on a delicate electronic component that might be adverselyaffected by pressure.

The gap (spacing) between the blade 1220 and the surface of the mask1210 is generally dependent upon the size of particles (not illustrated)in the solder paste 1202. Typically, the gap is 2-5 times the averageparticle size.

The blade 1220 suitably has a thickness of approximately 0.250 inches,is spaced approximately 5-7 mils from the surface of the mask 210, andis suitably formed of a material ranging from a very hard material suchas stainless steel to a relatively soft material such as 60 Shore Arubber. A suitable material is Ulon™.

Since the principal purpose of the flood blade 1220 is simply to pushsolder paste into the cells, its composition and end-profile (e.g., dullversus pointy) do not matter very much.

Preferably, the flood blade 1220 is inclined in the direction of travel,rather than straight up and down (as illustrated)—for example at anangle of 75 degrees (rather than 90 degrees, as illustrated) withrespect to the surface of the mask.

A second, “cleaning” blade 1230, such as a Permalex™ blade by TransitionAutomation SPK-PLX-1.5-9, is disposed so as to contact the mask 1210,and advances in the direction of the arrow 1222. In essence, thecleaning blade 1230 follows a suitable distance behind the flood blade1220, and performs “clean up” duty. By way of example, the distancebetween the two blades 1220 and 1230 is approximately 1″ (one inch)which is quite suitable for printing a mask for a 6 or 8 inch wafer.This distance between the blades 1220 and 1230 should be sufficient toallow room for the accumulation of paste left behind by the flood blade1220.

Since the cleaning blade 1230 need not perform a cell-filling function,it can have a low contact force (e.g., 2500 grams) with the surface ofthe mask 1210. As discussed above, a high contact force can beundesirable. And the non-compliance of the blade 1230 allows it to cleanthe surface of the mask without gouging (removing solder paste from) thealready-filled cells.

The blade 1230 is suitably spring steel or the like, then the tip orprinting edge is coated with a polyimide coating, then a final metalcoating is deposited. This as claimed by the manufacturer is the commonground between hard steel (no compliance requiring high pressures toobtain complete contact) and soft rubber that deflects into cell volumeand gouges (conforms too well)

The blade 1230 suitably has a thickness of 0.010 inches, is in contactwith the surface of the mask 1210, and is suitably formed of a materialranging from a very hard material such as stainless steel to arelatively hard material such as spring steel. The end of the blade 1230in contact with the mask 1210 and is specially coated to ensuring goodcleaning of the mask surface without gouging solder paste out of thecells.

The flood blade 1220 and the cleaning blade 1230 may move in unison, orindependently from one another. The may both be inclined in thedirection of travel. The flood blade 1220 is suitably of a plasticmaterial, and is spaced a distance equivalent to a few (e.g., 2-5)average solder paste particle sizes from the surface of the mask 1210.The cleaning blade 1230 is suitably of a metal material, and ispreferably thicker than the cross-dimension of a cell 1234. The floodblade 1220 and the cleaning blade 1230 are shown out-of-scale (not toscale), vis-a-vis the mask 1210, for illustrative clarity.

Therefore, the invention can generally be characterized as comprisingusing two dissimilar blades to fill cells of a mask (1210) with solderpaste (1202). The first blade (1220) is not in contact with the mask,and therefore “overfills” the cells. The second blade (1230) followsbehind (after, later) the first blade (1220) and removes excess solderpaste from the surface of the mask. The first blade (1220) exerts nodirect pressure on the mask. The second blade (1230) exerts very littlepressure on the surface of the mask. The first blade (1220) is of a widerange of materials. The second blade (1230) is preferably of anon-compliant material.

A person having ordinary skill in the art to which this invention mostnearly pertains will recognize that any suitable mechanical mechanism(e.g., actuators, etc.) can be used to control the movement of theblades (1220, 1230) across the surface of the mask (1210), and that theycan be moved in unison with one another, or independently from oneanother.

The two blades (1220, 1230), herein considered to be a “set” of blades,can be moved in unison, as discussed above, with the second blade (1230)trailing the first (1230) and moving in the same direction as the first(1220). The technique of the present invention has been found to bereliable for fully filling the cells of a mask, in only one pass.Alternatively, the second blade (1230) can be independently moved acrossthe surface of the mask, including in a different direction than thefirst blade (1220), including making more than one pass across the maskto ensure that the surface of mask is clean.

FIG. 13 illustrates an embodiment of a set of blades comprising a firstblade 1320 (compare 1220) and second blade 1330 (compare 230) forprinting a mask 1310 (compare 210). Profiles for the two blades 1320 and1330 are described. The flood blade 1320 is generally rectangular incross-section, having a leading edge (surface) 1322, a trailing edge(surface) 1324 which is generally parallel to the leading edge, and aside edge (surface) 1326 (comprising 1326 a,b,c) which is generallyperpendicular to the leading and trailing edges. In use, the side edge1326 is disposed opposing (facing) the mask 1310, but is not in contactwith the surface of the mask. (A non-wettable support surface, compare1208, is omitted, for illustrative clarity.)

The side edge 1326 is chamfered (beveled) so as to present a slopingsurface for pushing the solder paste (1202) down into the cells of themask when the blade 1320 is moved (left-to-right in the illustration)across the mask 1310. For example, from the trailing edge 1324, the sideedge 1326 has a first area 1326 a which is flat and perpendicular to thetrailing edge 1324 (and parallel to the mask 1310), followed by a secondarea 1326 b which forms approximately a 45-degree angle with the firstarea 1326 a, followed by a third area 1326 c which forms a steeper,approximately 60-degree angle with the first area (or, a shallow,approximately 30-degree angle with respect to the leading edge 1322).This “business end” of this blade 1320 is shown with a flat area 1326 aand compound bevel 1326 b,c at the junction of the side edge 1326 andthe leading edge 1322. The flat area 1326 a is preferably approximately75% of the overall blade thickness.

When the blade 320 is moved across a mask, with a glob of solder pastein from of it (see, e.g., FIG. 2), the 60-degree area 1326 c is thefirst to encounter the older paste (see FIG. 2) as the blade movesacross the mask. This angle, being less than 90-degrees, starts to pushthe solder paste down as the blade 1330 moves, exerting a mild downwardforce on the solder paste. (It should be understood that a similarresult could be obtained by tilting the entire blade 230 of FIG. 2forward 30-degrees from vertical.) The next, 45-degree area 1326 bfurther helps to push the solder paste down into the cell. (With a30-degree tilted blade, this area would be 15-degrees steeper.) Finally,the flat area 1326 a forces the solder paste into the cell.

In any case, the flood blade 1320 has at least one area which firstencounters the solder paste at an angle between flat (parallel to themask surface) and vertical (perpendicular to the mask surface), to startpushing (directing) the solder paste down into the cells, followed by asubstantially flat (parallel to the mask surface) area for finallypushing (forcing) the solder paste into the cells. The point is to fill(in this case, overfill) the cells of the mask in one pass, withoutrequiring exerting a lot of pressure on the mask (particularly if themask were atop a delicate electronic component).

FIG. 13 also illustrates an embodiment of a cleaning blade 1330 (compare1230). As mentioned above, the cleaning blade 1330 is preferably formedof a relatively non-compliant material, such as metal. The cleaningblade 1330 comes into contact with the mask 1310. An end portion 1332 ofthe cleaning blade 1320 preferably forms an approximately 45-degreeangle with the surface of the mask 1310. For example, between 30-degreesand 60-degrees, preferably approximately 45-degrees. The cleaning blade1330 could simply be one flat sheet of metal inclined at saidapproximately 45-degrees. However, in a set of blades moving in unisonacross a mask, the cleaning blade 1330 needs to “fit” behind the floodblade 1320. Therefore, the cleaning blade 1330 is suitably bent (folded)so that the angled end portion 1332 extends from a base portion 1334which extends substantially parallel to the flood blade 1320(perpendicular to the mask 1310). (Here, the end portion 1332 is shownat 45 degrees to the surface of the mask. The end portion 1332 should bebetween 30-60 degrees to the surface of the mask. The end portion 1332forms an obtuse angle with the base portion 1324.) It has been foundthat the base portion 1334 should be at least 2″ (two inches, 5centimeters) in length for filling a “normal” mask for ball bumping a6-8 inch wafer. This dimension was determined empirically. The “long”(quasi-cantilevered, compliantly-mounted) mounting of the cleaning bladeperforms well. It is believed that it creates a bit of compliance,avoiding “chatter” during the process of scraping excess solder pasteoff of the surface of the mask.

FIG. 14 illustrates an arrangement wherein two sets of blades are usedto expedite automatic mask printing. The drawing is merely illustrative,and is not to scale. A first set of blades comprises a flood blade 1420(compare 1320) and a cleaning blade 1430 (compare 1330). A second set ofblades comprises a flood blade 1440 (compare 1320) and a cleaning blade1450 (compare 1330). A mask 410 (compare 1310) is disposed between twoprint landing areas 1450 and 1460. (A non-wettable support surface,compare 1208, is omitted, for illustrative clarity.)

The first set of blades 1420/1430 is “parked” on the first print landingarea 1460. A glob of solder paste (compare 1202) is disposed in front ofthe flood blade 1420, on the first print landing area 1460. The firstset of blades 1420/1430 then advances across the mask 1410 (fromleft-to-right, as illustrated), towards the second print landing area1470, to fill the cells of the mask (to “print” the mask). The first setof blades continues to print, until it is entirely beyond the mask, anduntil the residual solder paste (that portion of the solder paste whichdid not make it into the cells) that is being pushed forward is on thesecond print landing area 1470. Then the first set of blades 420/430 canbe retracted, and repositioned on the first print landing area 1460.Meanwhile, the printed mask is taken away, and another, subsequent maskis positioned between the two print landing areas 1460 and 1470 to beprinted. The second set of blades 1440/1450 is the “mirror image” of thefirst set of blades, and prints the subsequent mask by pushing theresidual solder paste across the mask, from right-to-left (asillustrated). When finished, the residual solder paste that has beenpushed forward (to the left) by the second set of blades will be on thefirst print landing area 1460, and the second set of blades will returnto its starting position. A subsequent mask can then be printed by thefirst set of blades pushing this residual solder paste over thesubsequent mask onto the second print landing area, etc, so long asthere is an adequate supply of residual solder paste. In this manner,certain efficiencies of operation can be achieved.

CONCLUSION

In conclusion, the inventors offer the following general observationsfor successfully forming solder bumps (including balls and reflowableinterconnect structures) on substrates, particularly semiconductorwafers.

-   -   Closing (capturing) the cells is important because as the solder        heats to reflow temperatures some of the flux component outgases        to vapor, this vapor will expel solder material before the        reflow temperature is achieved and poor volume control will        result. The captured cell is also used to control or limit mask        warpage during heating. This mask warpage is contained to ensure        the proximity of the solder to the pad to be soldered.    -   Inverting, or partially inverting has been found to be often of        limited importance for small solder balls, because surface        tension forces tend to dominate the cell bump behavior. However,        inverting or partially inverting can be very important for        bumping large solder balls since it facilitates venting the        large volumes of gas produced by the greater amount of solder        paste present.    -   Using electromagnets (1028) to secure the mask carrier (1020) to        the chuck base (1014) can be very beneficial since the power        which is applied to the electromagnets can be controlled. For        example, during alignment of the mask to the wafer, the        electromagnets can be turned on slightly to produce a bit of        drag as the mask and wafer are aligned with one another.    -   The double blade printing process (e.g., FIG. 12) is important        because when printing low viscosity material into small mask        openings the material will tend to wet the top surface of the        mask opening and not allow the cell to fill properly. As it is        desirable to use low viscosity solder pastes to control particle        suspension uniformity, and provide sufficient flux for        soldering, this double blade printing process has proven to be a        very effective method of reliably filling small mask openings        over a wide range of solder paste materials.

Although the invention has been illustrated and described in detail inthe drawings and foregoing description, the same is to be considered asillustrative and not restrictive in character—it being understood thatonly preferred embodiments have been shown and described, and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. Undoubtedly, many other “variations” on the“themes” set forth hereinabove will occur to one having ordinary skillin the art to which the present invention most nearly pertains, and suchvariations are intended to be within the scope of the invention, asdisclosed herein.

1. Method for forming solder bumps on a substrate having a plurality ofpads on a surface thereof, comprising: providing a mask having a firstsurface and a second surface, and a plurality of cells extending fromthe first surface at least partially through the mask to the secondsurface thereof; filling the cells with solder paste; disposing asubstrate in a chuck assembly; disposing the mask on a surface of thesubstrate with the first surface of the mask adjacent to the surface ofthe substrate; urging the substrate into positive contact with the maskby allowing the substrate to flex under pressure so as to maintainsubstantially intimate contact between the first surface of the mask andthe surface of the substrate; applying a pressure plate to the secondsurface of the mask with sufficient force to capture the cells betweenthe plate and substrate and to limit mask warping during solder reflow;reflowing the solder paste; and separating the substrate from the mask.2. Method, according to claim 1, wherein: the solder bumps are solderballs.
 3. Method, according to claim 1, wherein: the substrate is asemiconductor wafer prior to singulation.
 4. Method, according to claim1, wherein: the chuck assembly includes a diaphragm; and furthercomprising: introducing a gas at positive pressure to cause thediaphragm to deflect upwards to urge the substrate into positive contactwith the mask.
 5. Method, according to claim 1, wherein the cells extendcompletely through the mask from the first surface thereof to the secondsurface thereof.
 6. Method, according to claim 5, further comprising:prior to reflowing, closing the cells of the mask.
 7. Method, accordingto claim 1, wherein the chuck assembly comprises a biased chuck assemblycapable of reorienting the substrate.
 8. Method, according to claim 7,wherein the step of reflowing the solder paste occurs while thesubstrate is in at least a partially inverted orientation.
 9. Method,according to claim 8, further comprising using the chuck assembly toshuttle the substrate to the partially inverted orientation having anangle of at least 135 degrees.
 10. Method, according to claim 9, furthercomprising using the chuck assembly to shuttle the substrate to thepartially inverted orientation having an angle of about 180 degrees. 11.Method, according to claim 1, wherein the step of reflowing the solderpaste includes operating the plate as a heater.